When using TDengine to store and query data, the most important part of the data is timestamp. Timestamp must be specified when creating and inserting data rows. Timestamp must follow the rules below:
...
...
@@ -18,52 +19,54 @@ Time precision in TDengine can be set by the `PRECISION` parameter when executin
```sql
CREATEDATABASEdb_namePRECISION'ns';
```
## Data Types
In TDengine, the data types below can be used when specifying a column or tag.
| 1 | TIMESTAMP | 8 | Default precision is millisecond, microsecond and nanosecond are also supported |
| 2 | INT | 4 | Integer, the value range is [-2^31, 2^31-1] |
| 3 |INT UNSIGNED|4 | Unsigned integer, the value range is [0, 2^31-1] |
| 3 | INT UNSIGNED| 4| unsigned integer, the value range is [0, 2^32-1]
| 4 | BIGINT | 8 | Long integer, the value range is [-2^63, 2^63-1] |
| 5 | BIGINT UNSIGNED | 8 | Unsigned long integer, the value range is [0, 2^63-1] |
| 5 | BIGINT UNSIGNED | 8 | unsigned long integer, the value range is [0, 2^64-1] |
| 6 | FLOAT | 4 | Floating point number, the effective number of digits is 6-7, the value range is [-3.4E38, 3.4E38] |
| 7 | DOUBLE | 8 | Double precision floating point number, the effective number of digits is 15-16, the value range is [-1.7E308, 1.7E308] |
| 8 | BINARY | User Defined | Single-byte string for ASCII visible characters. Length must be specified when defining a column or tag of binary type. The string length can be up to 16374 bytes. The string value must be quoted with single quotes. The literal single quote inside the string must be preceded with back slash like `\'`|
| 9 | SMALLINT | 2 | Short integer, the value range is [-32768, 32767] |
| 10 | SMALLINT UNSIGNED | 2 | Unsigned short integer, the value range is [0, 32767] |
| 11 | TINYINT | 1 | Single-byte integer, the value range is [-128, 127] |
| 12 | TINYINT UNSIGNED | 1 | Unsigned single-byte integer, the value range is [0, 127] |
| 13 | BOOL | 1 | Bool, the value range is {true, false} |
| 14 | NCHAR | User Defined| Multi-Byte string that can include multi byte characters like Chinese characters. Each character of NCHAR type consumes 4 bytes storage. The string value should be quoted with single quotes. Literal single quote inside the string must be preceded with backslash, like `\’`. The length must be specified when defining a column or tag of NCHAR type, for example nchar(10) means it can store at most 10 characters of nchar type and will consume fixed storage of 40 bytes. An error will be reported if the string value exceeds the length defined. |
| 8 | BINARY | User Defined | Single-byte string for ASCII visible characters. Length must be specified when defining a column or tag of binary type. |
| 9 | SMALLINT | 2 | Short integer, the value range is [-32768, 32767] |
| 10 | INT UNSIGNED| 2| unsigned integer, the value range is [0, 65535]|
| 11 | TINYINT | 1 | Single-byte integer, the value range is [-128, 127] |
| 12 | TINYINT UNSIGNED | 1 | unsigned single-byte integer, the value range is [0, 255] |
| 13 | BOOL | 1 | Bool, the value range is {true, false} |
| 14 | NCHAR | User Defined| Multi-Byte string that can include multi byte characters like Chinese characters. Each character of NCHAR type consumes 4 bytes storage. The string value should be quoted with single quotes. Literal single quote inside the string must be preceded with backslash, like `\’`. The length must be specified when defining a column or tag of NCHAR type, for example nchar(10) means it can store at most 10 characters of nchar type and will consume fixed storage of 40 bytes. An error will be reported if the string value exceeds the length defined. |
| 15 | JSON | | JSON type can only be used on tags. A tag of json type is excluded with any other tags of any other type |
| 16 | VARCHAR | User Defined| Alias of BINARY type |
| 16 | VARCHAR | User-defined | Alias of BINARY |
:::note
- TDengine is case insensitive and treats any characters in the sql command as lower case by default, case sensitive strings must be quoted with single quotes.
- Only ASCII visible characters are suggested to be used in a column or tag of BINARY type. Multi-byte characters must be stored in NCHAR type.
- Only ASCII visible characters are suggested to be used in a column or tag of BINARY type. Multi-byte characters must be stored in NCHAR type.
- The length of BINARY can be up to 16374 bytes. The string value must be quoted with single quotes. You must specify a length in bytes for a BINARY value, for example binary(20) for up to twenty single-byte characters. If the data exceeds the specified length, an error will occur. The literal single quote inside the string must be preceded with back slash like `\'`
- Numeric values in SQL statements will be determined as integer or float type according to whether there is decimal point or whether scientific notation is used, so attention must be paid to avoid overflow. For example, 9999999999999999999 will be considered as overflow because it exceeds the upper limit of long integer, but 9999999999999999999.0 will be considered as a legal float number.
:::
## Constants
TDengine supports constants of multiple data type.
| 1 | [{+ \| -}]123 | BIGINT | Numeric constants are treated as BIGINT type. The value will be truncated if it exceeds the range of BIGINT type. |
| 2 | 123.45 | DOUBLE | Floating number constants are treated as DOUBLE type. TDengine determines whether it's a floating number based on if decimal point or scientific notation is used. |
| 3 | 1.2E3 | DOUBLE | Constants in scientific notation are treated ad DOUBLE type. |
| 4 | 'abc' | BINARY | String constants enclosed by single quotes are treated as BINARY type. Its size is determined as the acutal length. Single quote itself can be included by preceding backslash, i.e. `\'`, in a string constant. |
| 5 | "abc" | BINARY | String constants enclosed by double quotes are treated as BINARY type. Its size is determined as the acutal length. Double quote itself can be included by preceding backslash, i.e. `\"`, in a string constant. |
| 6 | TIMESTAMP {'literal' \| "literal"} | TIMESTAMP | A string constant following `TIMESTAMP` keyword is treated as TIMESTAMP type. The string should be in the format of "YYYY-MM-DD HH:mm:ss.MS". Its time precision is same as that of the current database being used. |
| 8 | {'' \| "" \| '\t' \| "\t" \| ' ' \| " " \| NULL } | -- | NULL constant, it can be used for any type.|
| 1 | [{+ \| -}]123 | BIGINT | Integer literals are of type BIGINT. Data that exceeds the length of the BIGINT type is truncated. |
| 2 | 123.45 | DOUBLE | Floating-point literals are of type DOUBLE. Numeric values will be determined as integer or float type according to whether there is decimal point or whether scientific notation is used. |
| 3 | 1.2E3 | DOUBLE | Literals in scientific notation are of type DOUBLE. |
| 4 | 'abc' | BINARY | Content enclosed in single quotation marks is of type BINARY. The size of a BINARY is the size of the string in bytes. A literal single quote inside the string must be escaped with a backslash (\'). |
| 5 | 'abc' | BINARY | Content enclosed in double quotation marks is of type BINARY. The size of a BINARY is the size of the string in bytes. A literal double quote inside the string must be escaped with a backslash (\"). |
| 6 | TIMESTAMP {'literal' \| "literal"} | TIMESTAMP | The TIMESTAMP keyword indicates that the following string literal is interpreted as a timestamp. The string must be in YYYY-MM-DD HH:mm:ss.MS format. The precision is inherited from the database configuration. |
| 7 | {TRUE \| FALSE} | BOOL | Boolean literals are of type BOOL. |
| 8 | {'' \| "" \| '\t' \| "\t" \| ' ' \| " " \| NULL } | -- | The preceding characters indicate null literals. These can be used with any data type. |
:::note
- TDengine determines whether it's a floating number based on if decimal point or scientific notation is used. So whether the value is determined as overflow depends on both the value and the determined type. For example, 9999999999999999999 is determined as overflow because it exceeds the upper limit of BIGINT type, while 9999999999999999999.0 is considered as a valid floating number because it is within the range of DOUBLE type.
Numeric values will be determined as integer or float type according to whether there is decimal point or whether scientific notation is used, so attention must be paid to avoid overflow. For example, 9999999999999999999 will be considered as overflow because it exceeds the upper limit of long integer, but 9999999999999999999.0 will be considered as a legal float number.
- BUFFER: specifies the size (in MB) of the write buffer for each vnode. Enter a value between 3 and 16384. The default value is 96.
- CACHEMODEL: specifies how the latest data in subtables is stored in the cache. The default value is none.
- none: The latest data is not cached.
- last_row: The last row of each subtable is cached. This option significantly improves the performance of the LAST_ROW function.
- last_value: The last non-null value of each column in each subtable is cached. This option significantly improves the performance of the LAST function under normal circumstances, such as statements including the WHERE, ORDER BY, GROUP BY, and INTERVAL keywords.
- both: The last row of each subtable and the last non-null value of each column in each subtable are cached.
- CACHESIZE: specifies the amount (in MB) of memory used for subtable caching on each vnode. Enter a value between 1 and 65536. The default value is 1.
- COMP: specifies how databases are compressed. The default value is 2.
- 0: Compression is disabled.
- 1: One-pass compression is enabled.
- 2: Two-pass compression is enabled.
- DURATION: specifies the time period contained in each data file. After the time specified by this parameter has elapsed, TDengine creates a new data file to store incoming data. You can use m (minutes), h (hours), and d (days) as the unit, for example DURATION 100h or DURATION 10d. If you do not include a unit, d is used by default.
- WAL_FSYNC_PERIOD: specifies the interval (in milliseconds) at which data is written from the WAL to disk. This parameter takes effect only when the WAL parameter is set to 2. The default value is 3000. Enter a value between 0 and 180000. The value 0 indicates that incoming data is immediately written to disk.
- MAXROWS: specifies the maximum number of rows recorded in a block. The default value is 4096.
- MINROWS: specifies the minimum number of rows recorded in a block. The default value is 100.
- KEEP: specifies the time for which data is retained. Enter a value between 1 and 365000. The default value is 3650. The value of the KEEP parameter must be greater than or equal to the value of the DURATION parameter. TDengine automatically deletes data that is older than the value of the KEEP parameter. You can use m (minutes), h (hours), and d (days) as the unit, for example KEEP 100h or KEEP 10d. If you do not include a unit, d is used by default.
- PAGES: specifies the number of pages in the metadata storage engine cache on each vnode. Enter a value greater than or equal to 64. The default value is 256. The space occupied by metadata storage on each vnode is equal to the product of the values of the PAGESIZE and PAGES parameters. The space occupied by default is 1 MB.
- PAGESIZE: specifies the size (in KB) of each page in the metadata storage engine cache on each vnode. The default value is 4. Enter a value between 1 and 16384.
- PRECISION: specifies the precision at which a database records timestamps. Enter ms for milliseconds, us for microseconds, or ns for nanoseconds. The default value is ms.
- REPLICA: specifies the number of replicas that are made of the database. Enter 1 or 3. The default value is 1. The value of the REPLICA parameter cannot exceed the number of dnodes in the cluster.
- RETENTIONS: specifies the retention period for data aggregated at various intervals. For example, RETENTIONS 15s:7d,1m:21d,15m:50d indicates that data aggregated every 15 seconds is retained for 7 days, data aggregated every 1 minute is retained for 21 days, and data aggregated every 15 minutes is retained for 50 days. You must enter three aggregation intervals and corresponding retention periods.
- STRICT: specifies whether strong data consistency is enabled. The default value is off.
- on: Strong consistency is enabled and implemented through the Raft consensus algorithm. In this mode, an operation is considered successful once it is confirmed by half of the nodes in the cluster.
- off: Strong consistency is disabled. In this mode, an operation is considered successful when it is initiated by the local node.
- WAL_LEVEL: specifies whether fsync is enabled. The default value is 1.
- 1: WAL is enabled but fsync is disabled.
- 2: WAL and fsync are both enabled.
- VGROUPS: specifies the initial number of vgroups when a database is created.
- SINGLE_STABLE: specifies whether the database can contain more than one supertable.
- 0: The database can contain multiple supertables.
- 1: The database can contain only one supertable.
- WAL_RETENTION_PERIOD: specifies the time after which WAL files are deleted. This parameter is used for data subscription. Enter a time in seconds. The default value is 0. A value of 0 indicates that each WAL file is deleted immediately after its contents are written to disk. -1: WAL files are never deleted.
- WAL_RETENTION_SIZE: specifies the size at which WAL files are deleted. This parameter is used for data subscription. Enter a size in KB. The default value is 0. A value of 0 indicates that each WAL file is deleted immediately after its contents are written to disk. -1: WAL files are never deleted.
- WAL_ROLL_PERIOD: specifies the time after which WAL files are rotated. After this period elapses, a new WAL file is created. The default value is 0. A value of 0 indicates that a new WAL file is created only after the previous WAL file was written to disk.
- WAL_SEGMENT_SIZE: specifies the maximum size of a WAL file. After the current WAL file reaches this size, a new WAL file is created. The default value is 0. A value of 0 indicates that a new WAL file is created only after the previous WAL file was written to disk.
### Example Statement
```sql
createdatabaseifnotexistsdbvgroups10buffer10
```
The preceding SQL statement creates a database named db that has 10 vgroups and whose vnodes have a 10 MB cache.
1. KEEP specifies the number of days for which the data in the database will be retained. The default value is 3650 days, i.e. 10 years. The data will be deleted automatically once its age exceeds this threshold.
2. UPDATE specifies whether the data can be updated and how the data can be updated.
1. UPDATE set to 0 means update operation is not allowed. The update for data with an existing timestamp will be discarded silently and the original record in the database will be preserved as is.
2. UPDATE set to 1 means the whole row will be updated. The columns for which no value is specified will be set to NULL.
3. UPDATE set to 2 means updating a subset of columns for a row is allowed. The columns for which no value is specified will be kept unchanged.
3. The maximum length of database name is 33 bytes.
4. The maximum length of a SQL statement is 65,480 bytes.
5. Below are the parameters that can be used when creating a database
6. Please note that all of the parameters mentioned in this section are configured in configuration file `taos.cfg` on the TDengine server. If not specified in the `create database` statement, the values from taos.cfg are used by default. To override default parameters, they must be specified in the `create database` statement.
:::
The preceding SQL statement switches to the specified database. (If you connect to TDengine over the REST API, this statement does not take effect.)
## Show Current Configuration
## Drop a Database
```
SHOW VARIABLES;
DROP DATABASE [IF EXISTS] db_name
```
## Specify The Database In Use
The preceding SQL statement deletes the specified database. This statement will delete all tables in the database and destroy all vgroups associated with it. Exercise caution when using this statement.
```
USE db_name;
```
:::note
This way is not applicable when using a REST connection. In a REST connection the database name must be specified before a table or stable name. For e.g. to query the stable "meters" in database "test" the query would be "SELECT count(*) from test.meters"
## Change Database Configuration
:::
```sql
ALTERDATABASEdb_name[alter_database_options]
## Drop Database
alter_database_options:
alter_database_option...
```
DROP DATABASE [IF EXISTS] db_name;
alter_database_option:{
CACHEMODEL{'none'|'last_row'|'last_value'|'both'}
|CACHESIZEvalue
|WAL_LEVELvalue
|WAL_FSYNC_PERIODvalue
|KEEPvalue
}
```
:::note
All data in the database will be deleted too. This command must be used with extreme caution. Please follow your organization's data integrity, data backup, data security or any other applicable SOPs before using this command.
Other parameters cannot be modified after the database has been created.
:::
## Change Database Configuration
## View Databases
Some examples are shown below to demonstrate how to change the configuration of a database. Please note that some configuration parameters can be changed after the database is created, but some cannot. For details of the configuration parameters of database please refer to [Configuration Parameters](/reference/config/).
### View All Databases
```
ALTER DATABASE db_name COMP 2;
```
COMP parameter specifies whether the data is compressed and how the data is compressed.
```
ALTER DATABASE db_name REPLICA 2;
```
REPLICA parameter specifies the number of replicas of the database.
```
ALTER DATABASE db_name KEEP 365;
SHOW DATABASES;
```
KEEP parameter specifies the number of days for which the data will be kept.
### View the CREATE Statement for a Database
```
ALTER DATABASE db_name QUORUM 2;
SHOW CREATE DATABASE db_name;
```
QUORUM parameter specifies the necessary number of confirmations to determine whether the data is written successfully.
The preceding SQL statement can be used in migration scenarios. This command can be used to get the CREATE statement, which can be used in another TDengine instance to create the exact same database.
```
ALTER DATABASE db_name BLOCKS 100;
```
### View Database Configuration
BLOCKS parameter specifies the number of memory blocks used by each VNODE.
```
ALTER DATABASE db_name CACHELAST 0;
```sql
SHOWDATABASES\G;
```
CACHELAST parameter specifies whether and how the latest data of a sub table is cached.
The preceding SQL statement shows the value of each parameter for the specified database. One value is displayed per line.
:::tip
The above parameters can be changed using `ALTER DATABASE` command without restarting. For more details of all configuration parameters please refer to [Configuration Parameters](/reference/config/).
:::
## Delete Expired Data
## Show All Databases
```
SHOW DATABASES;
```
## Show The Create Statement of A Database
```
SHOW CREATE DATABASE db_name;
```sql
TRIMDATABASEdb_name;
```
This command is useful when migrating the data from one TDengine cluster to another. This command can be used to get the CREATE statement, which can be used in another TDengine instance to create the exact same database.
The preceding SQL statement deletes data that has expired and orders the remaining data in accordance with the storage configuration.
1. The first column of a table MUST be of type TIMESTAMP. It is automatically set as the primary key.
2. The maximum length of the table name is 192 bytes.
3. The maximum length of each row is 48k bytes, please note that the extra 2 bytes used by each BINARY/NCHAR column are also counted.
4. The name of the subtable can only consist of characters from the English alphabet, digits and underscore. Table names can't start with a digit. Table names are case insensitive.
5. The maximum length in bytes must be specified when using BINARY or NCHAR types.
6. Escape character "\`" can be used to avoid the conflict between table names and reserved keywords, above rules will be bypassed when using escape character on table names, but the upper limit for the name length is still valid. The table names specified using escape character are case sensitive. Only ASCII visible characters can be used with escape character.
6. Escape character "\`" can be used to avoid the conflict between table names and reserved keywords, above rules will be bypassed when using escape character on table names, but the upper limit for the name length is still valid. The table names specified using escape character are case sensitive.
For example \`aBc\` and \`abc\` are different table names but `abc` and `aBc` are same table names because they are both converted to `abc` internally.
Only ASCII visible characters can be used with escape character.
:::
**Parameter description**
1. COMMENT: specifies comments for the table. This parameter can be used with supertables, standard tables, and subtables.
2. WATERMARK: specifies the time after which the window is closed. The default value is 5 seconds. Enter a value between 0 and 15 minutes in milliseconds, seconds, or minutes. You can enter multiple values separated by commas (,). This parameter applies only to supertables and takes effect only when the RETENTIONS parameter has been specified for the database.
3. MAX_DELAY: specifies the maximum latency for pushing computation results. The default value is 15 minutes or the value of the INTERVAL parameter, whichever is smaller. Enter a value between 0 and 15 minutes in milliseconds, seconds, or minutes. You can enter multiple values separated by commas (,). Note: Retain the default value if possible. Configuring a small MAX_DELAY may cause results to be frequently pushed, affecting storage and query performance. This parameter applies only to supertables and takes effect only when the RETENTIONS parameter has been specified for the database.
4. ROLLUP: specifies aggregate functions to roll up. Rolling up a function provides downsampled results based on multiple axes. This parameter applies only to supertables and takes effect only when the RETENTIONS parameter has been specified for the database. You can specify only one function to roll up. The rollup takes effect on all columns except TS. Enter one of the following values: avg, sum, min, max, last, or first.
5. SMA: specifies functions on which to enable small materialized aggregates (SMA). SMA is user-defined precomputation of aggregates based on data blocks. Enter one of the following values: max, min, or sum This parameter can be used with supertables and standard tables.
6. TTL: specifies the time to live (TTL) for the table. If the period specified by the TTL parameter elapses without any data being written to the table, TDengine will automatically delete the table. Note: The system may not delete the table at the exact moment that the TTL expires. Enter a value in days. The default value is 0. Note: The TTL parameter has a higher priority than the KEEP parameter. If a table is marked for deletion because the TTL has expired, it will be deleted even if the time specified by the KEEP parameter has not elapsed. This parameter can be used with standard tables and subtables.
The tags for which no value is specified will be set to NULL.
The preceding SQL statement creates a subtable based on a supertable but specifies a subset of tags to use. Tags that are not included in this subset are assigned a null value.
This can be used to create a lot of tables in a single SQL statement while making table creation much faster.
You can create multiple subtables in a single SQL statement provided that all subtables use the same supertable. For performance reasons, do not create more than 3000 tables per statement.
## Modify a Table
:::info
```sql
ALTERTABLE[db_name.]tb_namealter_table_clause
alter_table_clause:{
alter_table_options
|ADDCOLUMNcol_namecolumn_type
|DROPCOLUMNcol_name
|MODIFYCOLUMNcol_namecolumn_type
|RENAMECOLUMNold_col_namenew_col_name
}
alter_table_options:
alter_table_option...
alter_table_option:{
TTLvalue
|COMMENT'string_value'
}
- Creating tables in batch must use a super table as a template.
- The length of single statement is suggested to be between 1,000 and 3,000 bytes for best performance.
```
:::
**More explanations**
You can perform the following modifications on existing tables:
1. ADD COLUMN: adds a column to the supertable.
2. DROP COLUMN: deletes a column from the supertable.
3. MODIFY COLUMN: changes the length of the data type specified for the column. Note that you can only specify a length greater than the current length.
4. RENAME COLUMN: renames a specified column in the table.
This is useful when migrating the data in one TDengine cluster to another one because it can be used to create the exact same tables in the target database.
If a table is created using a super table as template, the table definition can only be changed on the corresponding super table, and the change will be automatically applied to all the subtables created using this super table as template. For tables created in the normal way, the table definition can be changed directly on the table.
## View Tables
:::
### View All Tables
### Change Column Length
The following SQL statement shows all tables in the current database.
```
ALTER TABLE tb_name MODIFY COLUMN field_name data_type(length);
```sql
SHOWTABLES[LIKEtb_name_wildchar];
```
If the type of a column is variable length, like BINARY or NCHAR, this command can be used to change the length of the column.
### View the CREATE Statement for a Table
:::note
If a table is created using a super table as template, the table definition can only be changed on the corresponding super table, and the change will be automatically applied to all the subtables created using this super table as template. For tables created in the normal way, the table definition can be changed directly on the table.
```
SHOW CREATE TABLE tb_name;
```
:::
This command is useful in migrating data from one TDengine cluster to another because it can be used to create the exact same tables in the target database.
### Change Tag Value Of Sub Table
## View the Table Schema
```
ALTER TABLE tb_name SET TAG tag_name=new_tag_value;
```
This command can be used to change the tag value if the table is created using a super table as template.
- Each supertable can have a maximum of 4096 columns, including tags. The minimum number of columns is 3: a timestamp column used as the key, one tag column, and one data column.
- When you create a supertable, you can add comments to columns and tags.
- The TAGS keyword defines the tag columns for the supertable. The following restrictions apply to tag columns:
- A tag column can use the TIMESTAMP data type, but the values in the column must be fixed numbers. Timestamps including formulae, such as "now + 10s", cannot be stored in a tag column.
- The name of a tag column cannot be the same as the name of any other column.
- The name of a tag column cannot be a reserved keyword.
- Each supertable must contain between 1 and 128 tags. The total length of the TAGS keyword cannot exceed 16 KB.
- For more information about table parameters, see Create a Table.
The SQL statement of creating a STable is similar to that of creating a table, but a special column set named `TAGS` must be specified with the names and types of the tags.
:::info
The preceding SQL statement shows all supertables in the current TDengine database, including the name, creation time, number of columns, number of tags, and number of subtabels for each supertable.
1. A tag can be of type timestamp, since version 2.1.3.0, but its value must be fixed and arithmetic operations cannot be performed on it. Prior to version 2.1.3.0, tag types specified in TAGS could not be of type timestamp.
2. The tag names specified in TAGS should NOT be the same as other columns.
3. The tag names specified in TAGS should NOT be the same as any reserved keywords.(Please refer to [keywords](/taos-sql/keywords/)
4. The maximum number of tags specified in TAGS is 128, there must be at least one tag, and the total length of all tag columns should NOT exceed 16KB.
:::
## Drop STable
### View the CREATE Statement for a Supertable
```
DROP STable [IF EXISTS] stb_name;
SHOW CREATE STABLE stb_name;
```
All the subtables created using the deleted STable will be deleted automatically.
The preceding SQL statement can be used in migration scenarios. It returns the CREATE statement that was used to create the specified supertable. You can then use the returned statement to create an identical supertable on another TDengine database.
## Show All STables
## View the Supertable Schema
```
SHOW STableS [LIKE tb_name_wildcard];
DESCRIBE [db_name.]stb_name;
```
This command can be used to display the information of all STables in the current database, including name, creation time, number of columns, number of tags, and number of tables created using this STable.
## Show The Create Statement of A STable
## Drop STable
```
SHOW CREATE STable stb_name;
DROP STABLE [IF EXISTS] [db_name.]stb_name
```
This command is useful in migrating data from one TDengine cluster to another because it can be used to create the exact same STable in the target database.
Note: Deleting a supertable will delete all subtables created from the supertable, including all data within those subtables.
## Get STable Definition
## Modify a Supertable
```sql
ALTERSTABLE[db_name.]tb_namealter_table_clause
alter_table_clause:{
alter_table_options
|ADDCOLUMNcol_namecolumn_type
|DROPCOLUMNcol_name
|MODIFYCOLUMNcol_namecolumn_type
|ADDTAGtag_nametag_type
|DROPTAGtag_name
|MODIFYTAGtag_nametag_type
|RENAMETAGold_tag_namenew_tag_name
}
alter_table_options:
alter_table_option...
alter_table_option:{
COMMENT'string_value'
}
```
DESCRIBE stb_name;
```
## Change Columns Of STable
**More explanations**
Modifications to the table schema of a supertable take effect on all subtables within the supertable. You cannot modify the table schema of subtables individually. When you modify the tag schema of a supertable, the modifications automatically take effect on all of its subtables.
- ADD COLUMN: adds a column to the supertable.
- DROP COLUMN: deletes a column from the supertable.
- MODIFY COLUMN: changes the length of a BINARY or NCHAR column. Note that you can only specify a length greater than the current length.
- ADD TAG: adds a tag to the supertable.
- DROP TAG: deletes a tag from the supertable. When you delete a tag from a supertable, it is automatically deleted from all subtables within the supertable.
- MODIFY TAG: modifies the definition of a tag in the supertable. You can use this keyword to change the length of a BINARY or NCHAR tag column. Note that you can only specify a length greater than the current length.
- RENAME TAG: renames a specified tag in the supertable. When you rename a tag in a supertable, it is automatically renamed in all subtables within the supertable.
### Add A Column
### Add a Column
```
ALTER STable stb_name ADD COLUMN field_name data_type;
ALTER STABLE stb_name ADD COLUMN col_name column_type;
```
### Remove A Column
### Delete a Column
```
ALTER STable stb_name DROP COLUMN field_name;
ALTER STABLE stb_name DROP COLUMN col_name;
```
### Change Column Length
### Modify the Data Length
```
ALTER STable stb_name MODIFY COLUMN field_name data_type(length);
ALTER STABLE stb_name MODIFY COLUMN col_name data_type(length);
```
This command can be used to change (or more specifically, increase) the length of a column of variable length types, like BINARY or NCHAR.
## Change Tags of A STable
The preceding SQL statement changes the length of a BINARY or NCHAR data column. Note that you can only specify a length greater than the current length.
### Add A Tag
```
ALTER STable stb_name ADD TAG new_tag_name tag_type;
ALTER STABLE stb_name ADD TAG tag_name tag_type;
```
This command is used to add a new tag for a STable and specify the tag type.
The preceding SQL statement adds a tag of the specified type to the supertable. A supertable cannot contain more than 128 tags. The total length of all tags in a supertable cannot exceed 16 KB.
### Remove A Tag
```
ALTER STable stb_name DROP TAG tag_name;
ALTER STABLE stb_name DROP TAG tag_name;
```
The tag will be removed automatically from all the subtables, created using the super table as template, once a tag is removed from a super table.
The preceding SQL statement deletes a tag from the supertable. When you delete a tag from a supertable, it is automatically deleted from all subtables within the supertable.
### Change A Tag
```
ALTER STable stb_name CHANGE TAG old_tag_name new_tag_name;
ALTER STABLE stb_name RENAME TAG old_tag_name new_tag_name;
```
The tag name will be changed automatically for all the subtables, created using the super table as template, once a tag name is changed for a super table.
The preceding SQL statement renames a tag in the supertable. When you rename a tag in a supertable, it is automatically renamed in all subtables within the supertable.
### Change Tag Length
```
ALTER STable stb_name MODIFY TAG tag_name data_type(length);
ALTER STABLE stb_name MODIFY TAG tag_name data_type(length);
```
This command can be used to change (or more specifically, increase) the length of a tag of variable length types, like BINARY or NCHAR.
The preceding SQL statement changes the length of a BINARY or NCHAR tag column. Note that you can only specify a length greater than the current length. (Available in 2.1.3.0 and later versions)
### View a Supertable
You can run projection and aggregate SELECT queries on supertables, and you can filter by tag or column by using the WHERE keyword.
If you do not include an ORDER BY clause, results are returned by subtable. These results are not ordered. You can include an ORDER BY clause in your query to strictly order the results.
:::note
Changing tag values can be applied to only subtables. All other tag operations, like add tag, remove tag, however, can be applied to only STable. If a new tag is added for a STable, the tag will be added with NULL value for all its subtables.
All tag operations except for updating the value of a tag must be performed on the supertable and not on individual subtables. If you add a tag to an existing supertable, the tag is automatically added with a null value to all subtables within the supertable.
Single row or multiple rows specified with VALUES can be inserted into a specific table. For example:
1. All data writes must include a timestamp. With regard to timestamps, note the following:
A single row is inserted using the below statement.
2. The precision of a timestamp depends on its format. The precision configured for the database affects only timestamps that are inserted as long integers (UNIX time). Timestamps inserted as date and time strings are not affected. As an example, the timestamp 2021-07-13 16:16:48 is equivalent to 1626164208 in UNIX time. This UNIX time is modified to 1626164208000 for databases with millisecond precision, 1626164208000000 for databases with microsecond precision, and 1626164208000000000 for databases with nanosecond precision.
```sq;
3. If you want to insert multiple rows simultaneously, do not use the NOW function in the timestamp. Using the NOW function in this situation will cause multiple rows to have the same timestamp and prevent them from being stored correctly. This is because the NOW function obtains the current time on the client, and multiple instances of NOW in a single statement will return the same time.
The earliest timestamp that you can use when inserting data is equal to the current time on the server minus the value of the KEEP parameter. The latest timestamp that you can use when inserting data is equal to the current time on the server plus the value of the DURATION parameter. You can configure the KEEP and DURATION parameters when you create a database. The default values are 3650 days for the KEEP parameter and 10 days for the DURATION parameter.
**Syntax**
1. The USING clause automatically creates the specified subtable if it does not exist. If it's unknown whether the table already exists, the table can be created automatically while inserting using the SQL statement below. To use this functionality, a STable must be used as template and tag values must be provided. Any tags that you do not specify will be assigned a null value.
2. You can insert data into specified columns. Any columns in which you do not insert data will be assigned a null value.
3. The VALUES clause inserts one or more rows of data into a table.
4. The FILE clause inserts tags or data from a comma-separates values (CSV) file. Do not include headers in your CSV files.
5. A single INSERT statement can write data to multiple tables.
6. The INSERT statement is fully parsed before being executed, so that if any element of the statement fails, the entire statement will fail. For example, the following statement will not create a table because the latter part of the statement is invalid:
7. However, an INSERT statement that writes data to multiple subtables can succeed for some tables and fail for others. This situation is caused because vnodes perform write operations independently of each other. One vnode failing to write data does not affect the ability of other vnodes to write successfully.
## Insert a Record
Single row or multiple rows specified with VALUES can be inserted into a specific table. A single row is inserted using the below statement.
```sql
INSERTINTOd1001VALUES(NOW,10.2,219,0.32);
```
## Insert Multiple Records
Double rows are inserted using the below statement.
1. In the second example above, different formats are used in the two rows to be inserted. In the first row, the timestamp format is a date and time string, which is interpreted from the string value only. In the second row, the timestamp format is a long integer, which will be interpreted based on the database time precision.
2. When trying to insert multiple rows in a single statement, only the timestamp of one row can be set as NOW, otherwise there will be duplicate timestamps among the rows and the result may be out of expectation because NOW will be interpreted as the time when the statement is executed.
3. The oldest timestamp that is allowed is subtracting the KEEP parameter from current time.
4. The newest timestamp that is allowed is adding the DAYS parameter to current time.
:::
## Insert Into Specific Columns
## Write to a Specified Column
Data can be inserted into specific columns, either single row or multiple row, while other columns will be inserted as NULL value.
Data can be inserted into specific columns, either single row or multiple row, while other columns will be inserted as NULL value. The key (timestamp) cannot be null. For example:
If no columns are explicitly specified, all the columns must be provided with values, this is called "all column mode". The insert performance of all column mode is much better than specifying a subset of columns, so it's encouraged to use "all column mode" while providing NULL value explicitly for the columns for which no actual value can be provided.
:::
## Insert Into Multiple Tables
One or multiple rows can be inserted into multiple tables in a single SQL statement, with or without specifying specific columns.
One or multiple rows can be inserted into multiple tables in a single SQL statement, with or without specifying specific columns. For example:
If it's unknown whether the table already exists, the table can be created automatically while inserting using the SQL statement below. To use this functionality, a STable must be used as template and tag values must be provided.
If it's unknown whether the table already exists, the table can be created automatically while inserting using the SQL statement below. To use this functionality, a STable must be used as template and tag values must be provided. For example:
It's not necessary to provide values for all tags when creating tables automatically, the tags without values provided will be set to NULL.
It's not necessary to provide values for all tags when creating tables automatically, the tags without values provided will be set to NULL. For example:
Prior to version 2.0.20.5, when using `INSERT` to create tables automatically and specifying the columns, the column names must follow the table name immediately. From version 2.0.20.5, the column names can follow the table name immediately, also can be put between `TAGS` and `VALUES`. In the same SQL statement, however, these two ways of specifying column names can't be mixed.
:::
## Insert Rows From A File
Besides using `VALUES` to insert one or multiple rows, the data to be inserted can also be prepared in a CSV file with comma as separator and each field value quoted by single quotes. Table definition is not required in the CSV file. For example, if file "/tmp/csvfile.csv" contains the below data:
...
...
@@ -107,58 +119,13 @@ INSERT INTO d1001 FILE '/tmp/csvfile.csv';
## Create Tables Automatically and Insert Rows From File
From version 2.1.5.0, tables can be automatically created using a super table as template when inserting data from a CSV file, like below:
For SQL statement like `insert`, a stream parsing strategy is applied. That means before an error is found and the execution is aborted, the part prior to the error point has already been executed. Below is an experiment to help understand the behavior.
The output shows the value to be inserted is invalid. But `SHOW TABLES` proves that the table has been created automatically by the `INSERT` statement.
```
DB error: invalid SQL: 'a' (invalid timestamp) (0.039494s)
A query can be performed on some or all columns. Data and tag columns can all be included in the SELECT list.
## Wildcards
You can use an asterisk (\*) as a wildcard character to indicate all columns. For standard tables, the asterisk indicates only data columns. For supertables and subtables, tag columns are also included.
```sql
SELECT*FROMd1001;
```
Wildcard can be used with table name as prefix. Both SQL statements below have the same effect and return all columns.
You can use a table name as a prefix before an asterisk. For example, the following SQL statements both return all columns from the d1001 table:
```SQL
```sql
SELECT*FROMd1001;
SELECTd1001.*FROMd1001;
```
In a JOIN query, however, the results are different with or without a table name prefix. \* without table prefix will return all the columns of both tables, but \* with table name as prefix will return only the columns of that table.
However, in a JOIN query, using a table name prefix with an asterisk returns different results. In this case, querying * returns all data in all columns in all tables (not including tags), whereas using a table name prefix returns all data in all columns in the specified table only.
```
taos> SELECT * FROM d1001, d1003 WHERE d1001.ts=d1003.ts;
ts | current | voltage | phase | ts | current | voltage | phase |
The first of the preceding SQL statements returns all columns from the d1001 and d1003 tables, but the second of the preceding SQL statements returns all columns from the d1001 table only.
Wildcard \* can be used with some functions, but the result may be different depending on the function being used. For example, `count(*)` returns only one column, i.e. the number of rows; `first`, `last` and `last_row` return all columns of the selected row.
With regard to the other SQL functions that support wildcards, the differences are as follows:
`count(*)` only returns one column. `first`, `last`, and `last_row` return all columns.
You can query tag columns in supertables and subtables and receive results in the same way as querying data columns.
```sql
SELECTlocation,groupid,currentFROMd1001LIMIT2;
```
## Tags
### Distinct Values
Starting from version 2.0.14, tag columns can be selected together with data columns when querying sub tables. Please note however, that, wildcard \* cannot be used to represent any tag column. This means that tag columns must be specified explicitly like the example below.
The DISTINCT keyword returns only values that are different over one or more columns. You can use the DISTINCT keyword with tag columns and data columns.
```
taos> SELECT location, groupid, current FROM d1001 LIMIT 2;
The following SQL statement returns distinct values from a tag column:
## Get distinct values
```sql
SELECTDISTINCTtag_name[,tag_name...]FROMstb_name;
```
`DISTINCT` keyword can be used to get all the unique values of tag columns from a super table. It can also be used to get all the unique values of data columns from a table or subtable.
The following SQL statement returns distinct values from a data column:
When using `SELECT`, the column names in the result set will be the same as that in the select clause if `AS` is not used. `AS` can be used to rename the column names in the result set. For example
When using `SELECT`, the column names in the result set will be the same as that in the select clause if `AS` is not used. `AS` can be used to rename the column names in the result set. For example:
`AS` can't be used together with `first(*)`, `last(*)`, or `last_row(*)`.
## Implicit Columns
### Pseudocolumns
**TBNAME**
The TBNAME pseudocolumn in a supertable contains the names of subtables within the supertable.
`Select_exprs` can be column names of a table, or function expression or arithmetic expression on columns. The maximum number of allowed column names and expressions is 256. Timestamp and the corresponding tag names will be returned in the result set if `interval` or `group by tags` are used, and timestamp will always be the first column in the result set.
The following SQL statement returns all unique subtable names and locations within the meters supertable:
## Table List
```mysql
SELECT DISTINCT TBNAME, location FROM meters;
```
`FROM` can be followed by a number of tables or super tables, or can be followed by a sub-query. If no database is specified as current database in use, table names must be preceded with database name, like `power.d1001`.
Use the `INS_TAGS` system table in `INFORMATION_SCHEMA` to query the information for subtables in a supertable. For example, the following statement returns the name and tag values for each subtable in the `meters` supertable.
```SQL
SELECT * FROM power.d1001;
```mysql
SELECT table_name, tag_name, tag_type, tag_value FROM information_schema.ins_tags WHERE stable_name='meters';
```
has same effect as
The following SQL statement returns the number of subtables within the meters supertable.
```SQL
USE power;
SELECT * FROM d1001;
```mysql
SELECT COUNT(*) FROM (SELECT DISTINCT TBNAME FROM meters);
```
## Special Query
In the preceding two statements, only tags can be used as filtering conditions in the WHERE clause. For example:
Some special query functions can be invoked without `FROM` sub-clause. For example, the statement below can be used to get the current database in use.
**\_QSTART and \_QEND**
```
taos> SELECT DATABASE();
database() |
=================================
power |
Query OK, 1 row(s) in set (0.000079s)
```
The \_QSTART and \_QEND pseudocolumns contain the beginning and end of the time range of a query. If the WHERE clause in a statement does not contain valid timestamps, the time range is equal to [-2^63, 2^63 - 1].
If no database is specified upon logging in and no database is specified with `USE` after login, NULL will be returned by `select database()`.
The \_QSTART and \_QEND pseudocolumns cannot be used in a WHERE clause.
```
taos> SELECT DATABASE();
database() |
=================================
NULL |
Query OK, 1 row(s) in set (0.000184s)
```
**\_WSTART, \_WEND, and \_DURATION**
\_WSTART, \_WEND, and \_WDURATION pseudocolumns
The \_WSTART, \_WEND, and \_WDURATION pseudocolumns indicate the beginning, end, and duration of a window.
The statement below can be used to get the version of client or server.
These pseudocolumns can be used only in time window-based aggregations and must occur after the aggregation clause.
```
taos> SELECT CLIENT_VERSION();
client_version() |
===================
2.0.0.0 |
Query OK, 1 row(s) in set (0.000070s)
**\_c0 and \_ROWTS**
taos> SELECT SERVER_VERSION();
server_version() |
===================
2.0.0.0 |
Query OK, 1 row(s) in set (0.000077s)
In TDengine, the first column of all tables must be a timestamp. This column is the primary key of the table. The \_c0 and \_ROWTS pseudocolumns both represent the values of this column. These pseudocolumns enable greater flexibility and standardization. For example, you can use functions such as MAX and MIN with these pseudocolumns.
```sql
select_rowts,max(current)frommeters;
```
The statement below is used to check the server status. An integer, like `1`, is returned if the server status is OK, otherwise an error code is returned. This is compatible with the status check for TDengine from connection pool or 3rd party tools, and can avoid the problem of losing the connection from a connection pool when using the wrong heartbeat checking SQL statement.
## Query Objects
```
taos> SELECT SERVER_STATUS();
server_status() |
==================
1 |
Query OK, 1 row(s) in set (0.000074s)
`FROM` can be followed by a number of tables or super tables, or can be followed by a sub-query.
If no database is specified as current database in use, table names must be preceded with database name, for example, `power.d1001`.
taos> SELECT SERVER_STATUS() AS status;
status |
==============
1 |
Query OK, 1 row(s) in set (0.000081s)
```
You can perform INNER JOIN statements based on the primary key. The following conditions apply:
## \_block_dist
1. You can use FROM table list or an explicit JOIN clause.
2. For standard tables and subtables, you must specify an ON condition and the condition must be equivalent to the primary key.
3. For supertables, the ON condition must be equivalent to the primary key. In addition, the tag columns of the tables on which the INNER JOIN is performed must have a one-to-one relationship. You cannot specify an OR condition.
4. The tables that are included in a JOIN clause must be of the same type (supertable, standard table, or subtable).
5. You can include subqueries before and after the JOIN keyword.
6. You cannot include more than ten tables in a JOIN clause.
7. You cannot include a FILL clause and a JOIN clause in the same statement.
**Description**: Get the data block distribution of a table or STable.
## GROUP BY
```SQL title="Syntax"
SELECT _block_dist() FROM { tb_name | stb_name }
```
If you use a GROUP BY clause, the SELECT list can only include the following items:
**Restrictions**：No argument is allowed, where clause is not allowed
1. Constants
2. Aggregate functions
3. Expressions that are consistent with the expression following the GROUP BY clause
4. Expressions that include the preceding expression
**Sub Query**：Sub query or nested query are not supported
The GROUP BY clause groups each row of data by the value of the expression following the clause and returns a combined result for each group.
**Return value**: A string which includes the data block distribution of the specified table or STable, i.e. the histogram of rows stored in the data blocks of the table or STable.
The expressions in a GROUP BY clause can include any column in any table or view. It is not necessary that the expressions appear in the SELECT list.
The GROUP BY clause does not guarantee that the results are ordered. If you want to ensure that grouped data is ordered, use the ORDER BY clause.
**More explanation about above example**:
- Histogram about the rows stored in the data blocks of the table or STable: the value of rows for 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, and 99%
- Minimum number of rows stored in a data block, i.e. Min=[392(Rows)]
- Maximum number of rows stored in a data block, i.e. Max=[800(Rows)]
- Average number of rows stored in a data block, i.e. Avg=[666(Rows)]
- stddev of number of rows, i.e. Stddev=[2.17]
- Total number of rows, i.e. Rows[2000]
- Total number of data blocks, i.e. Blocks=[3]
- Total disk size consumed, i.e. Size=[5.440(Kb)]
- Compression ratio, which means the compressed size divided by original size, i.e. Comp=[0.23]
- Total number of rows in memory, i.e. RowsInMem=[0], which means no rows in memory
- The time spent on reading head file (to retrieve data block information), i.e. SeekHeaderTime=[1(us)], which means 1 microsecond.
## PARTITION BY
## Special Keywords in TAOS SQL
The PARTITION BY clause is a TDengine-specific extension to standard SQL. This clause partitions data based on the part_list and performs computations per partition.
-`TBNAME`： it is treated as a special tag when selecting on a super table, representing the name of subtables in that super table.
-`_c0`: represents the first column of a table or super table.
For more information, see TDengine Extensions.
## Tips
## ORDER BY
To get all the subtables and corresponding tag values from a super table:
The ORDER BY keyword orders query results. If you do not include an ORDER BY clause in a query, the order of the results can be inconsistent.
```SQL
SELECT TBNAME, location FROM meters;
```
You can specify integers after ORDER BY to indicate the order in which you want the items in the SELECT list to be displayed. For example, 1 indicates the first item in the select list.
You can specify ASC for ascending order or DESC for descending order.
You can also use the NULLS keyword to specify the position of null values. Ascending order uses NULLS LAST by default. Descending order uses NULLS FIRST by default.
## LIMIT
The LIMIT keyword controls the number of results that are displayed. You can also use the OFFSET keyword to specify the result to display first. `LIMIT` and `OFFSET` are executed after `ORDER BY` in the query execution. You can include an offset in a LIMIT clause. For example, LIMIT 5 OFFSET 2 can also be written LIMIT 2, 5. Both of these clauses display the third through the seventh results.
In a statement that includes a PARTITON BY clause, the LIMIT keyword is performed on each partition, not on the entire set of results.
## SLIMIT
The SLIMIT keyword is used with a PARTITION BY clause to control the number of partitions that are displayed. You can include an offset in a SLIMIT clause. For example, SLIMIT 5 OFFSET 2 can also be written LIMIT 2, 5. Both of these clauses display the third through the seventh partitions.
Note: If you include an ORDER BY clause, only one partition can be displayed.
## Special Query
To get the number of sub tables in a super table：
Some special query functions can be invoked without `FROM` sub-clause.
```SQL
SELECT COUNT(TBNAME) FROM meters;
## Obtain Current Database
The following SQL statement returns the current database. If a database has not been specified on login or with the `USE` command, a null value is returned.
```sql
SELECTDATABASE();
```
Only filter on `TAGS` are allowed in the `where` clause for above two query statements. For example:
taos> SELECT COUNT(tbname) FROM meters WHERE groupId > 2;
count(tbname) |
========================
2 |
Query OK, 1 row(s) in set (0.001091s)
## Obtain Server Status
The following SQL statement returns the status of the TDengine server. An integer indicates that the server is running normally. An error code indicates that an error has occurred. This statement can also detect whether a connection pool or third-party tool is connected to TDengine properly. By using this statement, you can ensure that connections in a pool are not lost due to an incorrect heartbeat detection statement.
```sql
SELECTSERVER_STATUS();
```
- Wildcard \* can be used to get all columns, or specific column names can be specified. Arithmetic operation can be performed on columns of numerical types, columns can be renamed in the result set.
- Arithmetic operation on columns can't be used in where clause. For example, `where a*2>6;` is not allowed but `where a>6/2;` can be used instead for the same purpose.
- Arithmetic operation on columns can't be used as the objectives of select statement. For example, `select min(2*a) from t;` is not allowed but `select 2*min(a) from t;` can be used instead.
- Logical operation can be used in `WHERE` clause to filter numeric values, wildcard can be used to filter string values.
- Result sets are arranged in ascending order of the first column, i.e. timestamp, but it can be controlled to output as descending order of timestamp. If `order by` is used on other columns, the result may not be as expected. By the way, \_c0 is used to represent the first column, i.e. timestamp.
-`LIMIT` parameter is used to control the number of rows to output. `OFFSET` parameter is used to specify from which row to output. `LIMIT` and `OFFSET` are executed after `ORDER BY` in the query execution. A simple tip is that `LIMIT 5 OFFSET 2` can be abbreviated as `LIMIT 2, 5`.
- What is controlled by `LIMIT` is the number of rows in each group when `GROUP BY` is used.
-`SLIMIT` parameter is used to control the number of groups when `GROUP BY` is used. Similar to `LIMIT`, `SLIMIT 5 OFFSET 2` can be abbreviated as `SLIMIT 2, 5`.
- ">>" can be used to output the result set of `select` statement to the specified file.
### Obtain Current Time
## Where
```sql
SELECTNOW();
```
Logical operations in below table can be used in the `where` clause to filter the resulting rows.
### Obtain Current Date
| **Operation** | **Note** | **Applicable Data Types** |
- Operator `<\>` is equal to `!=`, please note that this operator can't be used on the first column of any table, i.e.timestamp column.
- Operator `like` is used together with wildcards to match strings
- '%' matches 0 or any number of characters, '\_' matches any single ASCII character.
-`\_` is used to match the \_ in the string.
- The maximum length of wildcard string is 100 bytes from version 2.1.6.1 (before that the maximum length is 20 bytes). `maxWildCardsLength` in `taos.cfg` can be used to control this threshold. A very long wildcard string may slowdown the execution performance of `LIKE` operator.
-`AND` keyword can be used to filter multiple columns simultaneously. AND/OR operation can be performed on single or multiple columns from version 2.3.0.0. However, before 2.3.0.0 `OR` can't be used on multiple columns.
- For timestamp column, only one condition can be used; for other columns or tags, `OR` keyword can be used to combine multiple logical operators. For example, `((value > 20 AND value < 30) OR (value < 12))`.
- From version 2.3.0.0, multiple conditions can be used on timestamp column, but the result set can only contain single time range.
- From version 2.0.17.0, operator `BETWEEN AND` can be used in where clause, for example `WHERE col2 BETWEEN 1.5 AND 3.25` means the filter condition is equal to "1.5 ≤ col2 ≤ 3.25".
- From version 2.1.4.0, operator `IN` can be used in the where clause. For example, `WHERE city IN ('California.SanFrancisco', 'California.SanDiego')`. For bool type, both `{true, false}` and `{0, 1}` are allowed, but integers other than 0 or 1 are not allowed. FLOAT and DOUBLE types are impacted by floating point precision errors. Only values that match the condition within the tolerance will be selected. Non-primary key column of timestamp type can be used with `IN`.
- From version 2.3.0.0, regular expression is supported in the where clause with keyword `match` or `nmatch`. The regular expression is case insensitive.
```sql
SELECTTIMEZONE();
```
## Regular Expression
### Syntax
```SQL
```txt
WHERE (column|tbname) **match/MATCH/nmatch/NMATCH** _regex_
```
### Specification
The regular expression being used must be compliant with POSIX specification, please refer to[Regular Expressions](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html).
TDengine supports POSIX regular expression syntax. For more information, see[Regular Expressions](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html).
### Restrictions
Regular expression can be used against only table names, i.e. `tbname`, and tags of binary/nchar types, but can't be used against data columns.
Regular expression filtering is supported only on table names (TBNAME), BINARY tags, and NCHAR tags. Regular expression filtering cannot be performed on data columns.
The maximum length of regular expression string is 128 bytes. Configuration parameter `maxRegexStringLen` can be used to set the maximum allowed regular expression. It's a configuration parameter on the client side, and will take effect after restarting the client.
A regular expression string cannot exceed 128 bytes. You can configure this value by modifying the maxRegexStringLen parameter on the TDengine Client. The modified value takes effect when the client is restarted.
## JOIN
From version 2.2.0.0, inner join is fully supported in TDengine. More specifically, the inner join between table and table, between STable and STable, and between sub query and sub query are supported.
TDengine supports natural joins between supertables, between standard tables, and between subqueries. The difference between natural joins and inner joins is that natural joins require that the fields being joined in the supertables or standard tables must have the same name. Data or tag columns must be joined with the equivalent column in another table.
Only primary key, i.e. timestamp, can be used in the join operation between table and table. For example:
For standard tables, only the timestamp (primary key) can be used in join operations. For example:
```sql
SELECT*
...
...
@@ -356,25 +319,26 @@ FROM temp_tb_1 t1, pressure_tb_1 t2
WHEREt1.ts=t2.ts
```
In the join operation between STable and STable, besides the primary key, i.e. timestamp, tags can also be used. For example:
For supertables, tags as well as timestamps can be used in join operations. For example:
Similarly, join operations can be performed on the result set of multiple sub queries.
Similarly, join operations can be performed on the result sets of multiple subqueries.
:::note
Restrictions on join operation:
- The number of tables or STables in a single join operation can't exceed 10.
-`FILL` is not allowed in the query statement that includes JOIN operation.
- Arithmetic operation is not allowed on the result set of join operation.
-`GROUP BY` is not allowed on a part of tables that participate in join operation.
-`OR` can't be used in the conditions for join operation
- join operation can't be performed on data columns, i.e. can only be performed on tags or primary key, i.e. timestamp
The following restriction apply to JOIN statements:
- The number of tables or supertables in a single join operation cannot exceed 10.
-`FILL` cannot be used in a JOIN statement.
- Arithmetic operations cannot be performed on the result sets of join operation.
-`GROUP BY` is not allowed on a segment of the tables that participate in a join operation.
-`OR` cannot be used in the conditions for join operation
- Join operation can be performed only on tags or timestamps. You cannot perform a join operation on data columns.
:::
...
...
@@ -384,7 +348,7 @@ Nested query is also called sub query. This means that in a single SQL statement
From 2.2.0.0, unassociated sub query can be used in the `FROM` clause. Unassociated means the sub query doesn't use the parameters in the parent query. More specifically, in the `tb_name_list` of `SELECT` statement, an independent SELECT statement can be used. So a complete nested query looks like:
```SQL
```
SELECT ... FROM (SELECT ... FROM ...) ...;
```
...
...
@@ -408,42 +372,42 @@ SELECT ... FROM (SELECT ... FROM ...) ...;
## UNION ALL
```SQL title=Syntax
```txt title=Syntax
SELECT ...
UNION ALL SELECT ...
[UNION ALL SELECT ...]
```
`UNION ALL` operator can be used to combine the result set from multiple select statements as long as the result set of these select statements have exactly the same columns. `UNION ALL` doesn't remove redundant rows from multiple result sets. In a single SQL statement, at most 100 `UNION ALL` can be supported.
TDengine supports the `UNION ALL` operation. `UNION ALL` operator can be used to combine the result set from multiple select statements as long as the result set of these select statements have exactly the same columns. `UNION ALL` doesn't remove redundant rows from multiple result sets. In a single SQL statement, at most 100 `UNION ALL` can be supported.
The rows in the past one hour in `tb1` can be selected using below SQL statement:
```SQL
```
SELECT * FROM tb1 WHERE ts >= NOW - 1h;
```
The rows between 2018-06-01 08:00:00.000 and 2018-06-02 08:00:00.000 and col3 ends with 'nny' can be selected in the descending order of timestamp using below SQL statement:
```SQL
```
SELECT * FROM tb1 WHERE ts > '2018-06-01 08:00:00.000' AND ts <= '2018-06-02 08:00:00.000' AND col3 LIKE '%nny' ORDER BY ts DESC;
```
The sum of col1 and col2 for rows later than 2018-06-01 08:00:00.000 and whose col2 is bigger than 1.2 can be selected and renamed as "complex", while only 10 rows are output from the 5th row, by below SQL statement:
```SQL
```
SELECT (col1 + col2) AS 'complex' FROM tb1 WHERE ts > '2018-06-01 08:00:00.000' AND col2 > 1.2 LIMIT 10 OFFSET 5;
```
The rows in the past 10 minutes and whose col2 is bigger than 3.14 are selected and output to the result file `/home/testoutput.csv` with below SQL statement:
```SQL
```
SELECT COUNT(*) FROM tb1 WHERE ts >= NOW - 10m AND col2 > 3.14 >> /home/testoutput.csv;
@@ -4,8 +4,7 @@ description: "Delete data from table or Stable"
title: Delete Data
---
TDengine provides the functionality of deleting data from a table or STable according to specified time range, it can be used to cleanup abnormal data generated due to device failure. Please be noted that this functionality is only available in Enterprise version, please refer to [TDengine Enterprise Edition](https://tdengine.com/products#enterprise-edition-link)
TDengine provides the functionality of deleting data from a table or STable according to specified time range, it can be used to cleanup abnormal data generated due to device failure.
**Description:** Delete data from a table or STable
**Parameters:**
- `db_name`: Optional parameter, specifies the database in which the table exists; if not specified, the current database will be used.
- `tb_name`: Mandatory parameter, specifies the table name from which data will be deleted, it can be normal table, subtable or STable.
- `condition`: Optional parameter, specifies the data filter condition. If no condition is specified all data will be deleted, so please be cautions to delete data without any condition. The condition used here is only applicable to the first column, i.e. the timestamp column. If the table is a STable, the condition is also applicable to tag columns.
- `condition`: Optional parameter, specifies the data filter condition. If no condition is specified all data will be deleted, so please be cautions to delete data without any condition. The condition used here is only applicable to the first column, i.e. the timestamp column.
**More Explanations:**
The data can't be recovered once deleted, so please be cautious to use the functionality of deleting data. It's better to firstly make sure the data to be deleted using `select` then execute `delete`.
The data can't be recovered once deleted, so please be cautious to use the functionality of deleting data. It's better to firstly make sure the data to be deleted using `select` then execute `delete`.
**Example:**
`meters` is a STable, in which `groupid` is a tag column of int type. Now we want to delete the data older than 2021-10-01 10:40:00.100 and `groupid` is 1. The SQL for this purpose is like below:
`meters` is a STable, in which `groupid` is a tag column of int type. Now we want to delete the data older than 2021-10-01 10:40:00.100. You can perform this action by running the following SQL statement:
```sql
delete from meters where ts < '2021-10-01 10:40:00.100' and groupid=1 ;
delete from meters where ts < '2021-10-01 10:40:00.100' ;
**Description**: The logarithm of a specific field with `base` as the radix. If `base` parameter is ignored, natural logarithm of the field is returned.
**Description**: The logarithm of a specific field with `base` as the radix. If you do not enter a base, the natural logarithm of the field is returned.
**Return value type**: Double
**Return value type**: DOUBLE.
**Applicable data types**: Numeric
**Applicable data types**: Numeric types.
**Nested query**: It can be used in both the outer query and inner query in a nested query.
**Applicable table types**: table, STable.
**Applicable table types**: standard tables and supertables
**Applicable nested query**: Inner query and Outer query.
**Usage**: This function can only be used on data columns. It can be used with selection and projection functions but not with aggregation functions.
**Description**: The concatenation result of two or more strings.
**Description**: The concatenation result of two or more strings
**Return value type**: If all input strings are VARCHAR type, the result is VARCHAR type too. If any one of input strings is NCHAR type, then the result is NCHAR. If input strings contain NULL value, the result is NULL.
**Return value type**: If the concatenated strings are VARCHARs, the result is a VARCHAR. If the concatenated strings are NCHARs, the result is an NCHAR. If an input value is null, the result is null.
**Applicable data types**: VARCHAR, NCHAR. At least 2 input strings are required, and at most 8 input strings are allowed.
**Applicable data types**: VARCHAR and NCHAR You can concatenate between 2 and 8 strings.
**Applicable table types**: table, STable.
**Nested query**: It can be used in both the outer query and inner query in a nested query.
**Applicable nested query**: Inner query and Outer query.
**Applicable table types**: standard tables and supertables
**Description**: The concatenation result of two or more strings with separator.
**Description**: The concatenation result of two or more strings with separator
**Return value type**: If all input strings are VARCHAR type, the result is VARCHAR type too. If any one of input strings is NCHAR type, then the result is NCHAR. If input strings contain NULL value, the result is NULL.
**Return value type**: If the concatenated strings are VARCHARs, the result is a VARCHAR. If the concatenated strings are NCHARs, the result is an NCHAR. If an input value is null, the result is null.
**Applicable data types**: VARCHAR, NCHAR. At least 3 input strings are required, and at most 9 input strings are allowed.
**Applicable data types**: VARCHAR and NCHAR You can concatenate between 3 and 9 strings.
**Applicable table types**: table, STable.
**Nested query**: It can be used in both the outer query and inner query in a nested query.
**Applicable nested query**: Inner query and Outer query.
**Applicable table types**: standard tables and supertables
**Description**: The sub-string starting from `pos` with length of `len` from the original string `str`.
**Return value type**: Same as input type.
**Description**: The sub-string starting from `pos` with length of `len` from the original string `str` - If `len` is not specified, it means from `pos` to the end.
**Applicable data types**: VARCHAR, NCHAR.
**Return value type**: Same as input
**Applicable table types**: table, STable.
**Applicable data types**: VARCHAR and NCHAR Parameter `pos` can be an positive or negative integer; If it's positive, the starting position will be counted from the beginning of the string; if it's negative, the starting position will be counted from the end of the string.
**Applicable nested query**: Inner query and Outer query.
**Nested query**: It can be used in both the outer query and inner query in a nested query.
**More explanations**:
**Applicable table types**: table, STable
- If the input is NULL, the output is NULL
- Parameter `pos` can be an positive or negative integer; If it's positive, the starting position will be counted from the beginning of the string; if it's negative, the starting position will be counted from the end of the string.
- If `len` is not specified, it means from `pos` to the end of string.
**Description**: Used for type casting. Convert `expression` to the type specified by `type_name`.
**Description**: Convert the input data `expression` into the type specified by `type_name`. This function can be used only in SELECT statements.
**Return value type**: The type specified by parameter `type_name`.
**Return value type**: The type specified by parameter `type_name`
**Applicable data types**: All data types except JSON
**Nested query**: It can be used in both the outer query and inner query in a nested query.
**Applicable data types**: `expression` can be any data type except for JSON.
**Applicable table types**: standard tables and supertables
**More explanations**:
- Error will be reported for unsupported type casting.
- Error will be reported for unsupported type casting
- Some values of some supported data types may not be casted, below are known issues:
1）When casting VARCHAR/NCHAR to BIGINT/BIGINT UNSIGNED, some characters may be treated as illegal, for example "a" may be converted to 0.
2）When casting to numeric type, if converted result is out of range the destination data type can hold, overflow may occur and casting behavior is undefined.
3) When casting to VARCHAR/NCHAR type, if converted string length exceeds the length specified in `type_name`, the result will be truncated. (e.g. CAST("abcd" as BINARY(2)) will return string "ab").
1. Some strings cannot be converted to numeric values. For example, the string `a` may be converted to `0`. However, this does not produce an error.
2. If a converted numeric value is larger than the maximum size for the specified type, an overflow will occur. However, this does not produce an error.
3. If a converted string value is larger than the maximum size for the specified type, the output value will be truncated. However, this does not produce an error.
#### TO_ISO8601
...
...
@@ -434,18 +421,22 @@ SELECT CAST(expression AS type_name) FROM { tb_name | stb_name } [WHERE clause]
**Description**: The ISO8601 date/time format converted from a UNIX timestamp, with timezone attached. `timezone` parameter allows attaching any customized timezone string to the output format. If `timezone` parameter is not specified, the timezone information of client side system will be attached.
**Description**: The ISO8601 date/time format converted from a UNIX timestamp, plus the timezone. You can specify any time zone with the timezone parameter. If you do not enter this parameter, the time zone on the client is used.
**Return value type**: VARCHAR.
**Return value type**: VARCHAR
**Applicable data types**: INTEGER, TIMESTAMP.
**Applicable data types**: Integers and timestamps
**Applicable table types**: table, STable.
**Nested query**: It can be used in both the outer query and inner query in a nested query.
**Applicable table types**: standard tables and supertables
**More explanations**:
- If the input is INTEGER represents UNIX timestamp, the precision of the returned value is determined by the digits of the input integer.
- If the input is of TIMESTAMP type, The precision of the returned value is same as the precision set for the current database in use.
- You can specify a time zone in the following format: [z/Z, +/-hhmm, +/-hh, +/-hh:mm]。 For example, TO_ISO8601(1, "+00:00").
- If the input is a UNIX timestamp, the precision of the returned value is determined by the digits of the input timestamp
- If the input is a column of TIMESTAMP type, the precision of the returned value is same as the precision set for the current data base in use
**Description**: Convert a JSON string to a JSON body.
**Description**: Converts a string into JSON.
**Return value type**: JSON.
**Return value type**: JSON
**Applicable data types**: JSON string, in the format like '{ "literal" : literal }'. '{}' is NULL value. keys in the string must be string constants, values can be constants of numeric types, bool, string or NULL. Escaping characters are not allowed in the JSON string.
**Applicable data types**: JSON strings in the form `{"literal": literal}`. `{}` indicates a null value. The key must be a string literal. The value can be a numeric literal, string literal, Boolean literal, or null literal. str_literal cannot include escape characters.
**Applicable table types**: table, STable.
**Nested query**: It can be used in both the outer query and inner query in a nested query.
**Applicable table types**: table, STable
**Applicable nested query**: Inner query and Outer query.
**Description**: UNIX timestamp converted from a string of date/time format.
**Description**: UNIX timestamp converted from a string of date/time format
**Return value type**: BIGINT.
**Return value type**: BIGINT
**Applicable data types**: VARCHAR, NCHAR.
**Applicable column types**: VARCHAR and NCHAR
**Applicable table types**: table, STable.
**Nested query**: It can be used in both the outer query and inner query in a nested query.
**Applicable table types**: standard tables and supertables
**More explanations**:
- The input string must be compatible with ISO8601/RFC3339 standard, NULL will be returned if the string cannot be converted.
- The precision of the returned timestamp is same as the precision set for the current database in use.
- The input string must be compatible with ISO8601/RFC3339 standard, NULL will be returned if the string can't be converted
- The precision of the returned timestamp is same as the precision set for the current data base in use
### Time and Date Functions
### DateTime Functions
These functions perform operations on times and dates.
DateTime functions applied to timestamp data. NOW(), TODAY() and TIMEZONE() are executed only once even though they may occur multiple times in a single SQL statement.
All functions that return the current time, such as `NOW`, `TODAY`, and `TIMEZONE`, are calculated only once per statement even if they appear multiple times.
#### NOW
...
...
@@ -494,61 +491,66 @@ SELECT select_expr FROM { tb_name | stb_name } WHERE ts_col cond_operatior NOW()
INSERTINTOtb_nameVALUES(NOW(),...);
```
**Description**: The current time of the client side system.
**Description**: The current time of the client side system
**Return value type**: TIMESTAMP.
**Return value type**: TIMESTAMP
**Applicable data types**: TIMESTAMP only if used in WHERE/INSERT clause.
**Applicable column types**: TIMESTAMP only
**Applicable table types**: table, STable.
**Applicable table types**: standard tables and supertables
**Nested query**: It can be used in both the outer query and inner query in a nested query.
**More explanations**:
- Addition and Subtraction operation with time duration can be performed, for example NOW() + 1s, the time unit can be one of the followings:
- The precision of the returned timestamp is same as the precision set for the current data base in use
## Aggregate Functions
Aggregate functions return single result row for each group in the query result set. Groups are determined by `GROUP BY` clause or time window clause if they are used; or the whole result is considered a group if neither of them is used.
Aggregate functions return one row per group. You can use windows or GROUP BY to group data. Otherwise, the entire query is considered a single group.
TDengine supports the following aggregate functions:
**Description**: Similar to `PERCENTILE`, but a approximated result is returned.
**Return value type**: DOUBLE.
**Description**: Similar to `PERCENTILE`, but a simulated result is returned
**Applicable data types**: Numeric types.
**Return value type**: DOUBLE
**Applicable table types**: table, STable.
**Applicable data types**: Numeric
**More explanations**
**Applicable table types**: standard tables and supertables
**Explanations**：
- _P_ is in range [0,100], when _P_ is 0, the result is same as using function MIN; when _P_ is 100, the result is same as function MAX.
-**algo_type** can only be input as `default` or `t-digest`, if it's not specified `default` will be used, i.e. `apercentile(column_name, 50)` is same as `apercentile(column_name, 50, "default")`.
- If `default` is used, histogram based algorithm is used for calculation. If `t-digest` is used, `t-digest` sampling algorithm is used to calculate the result.
**Nested query**: It can be used in both the outer query and inner query in a nested query.
-`algo_type` can only be input as `default` or `t-digest` Enter `default` to use a histogram-based algorithm. Enter `t-digest` to use the t-digest algorithm to calculate the approximation of the quantile. `default` is used by default.
### AVG
```
```sql
SELECTAVG(field_name)FROMtb_name[WHEREclause];
```
**Description**: Get the average value of a column in a table or STable.
**Description**: The average value of the specified fields.
**Return value type**: DOUBLE
**Return value type**: DOUBLE.
**Applicable data types**: Numeric
**Applicable data types**: Numeric type.
**Applicable table types**: standard tables and supertables
**Description**：`elapsed` function can be used to calculate the continuous time length in which there is valid data. If it's used with `INTERVAL` clause, the returned result is the calcualted time length within each time window. If it's used without `INTERVAL` caluse, the returned result is the calculated time duration within the specified time range. Please be noted that the calculated time duration is in the specified `time_unit`.
**Description**：`elapsed` function can be used to calculate the continuous time length in which there is valid data. If it's used with `INTERVAL` clause, the returned result is the calcualted time length within each time window. If it's used without `INTERVAL` caluse, the returned result is the calculated time length within the specified time range. Please be noted that the return value of `elapsed` is the number of `time_unit` in the calculated time length.
**Return value type**：DOUBLE.
**Return value type**: Double if the input value is not NULL;
**Applicable data type**：TIMESTAMP.
**Return value type**: TIMESTAMP
**Applicable tables**: table, STable, outter in nested query.
**Applicable tables**: table, STable, outter in nested query
**Explanations**：
-`field_name` parameter can only be the first column of a table, i.e. timestamp primary key.
- The minimum value of `time_unit` is the time precision of the database. If `time_unit` is not specified, the time precision of the database is used as the default ime unit.
- The minimum value of `time_unit` is the time precision of the database. If `time_unit` is not specified, the time precision of the database is used as the default time unit. Time unit specified by `time_unit` can be:
- It can be used with `INTERVAL` to get the time valid time length of each time window. Please be noted that the return value is same as the time window for all time windows except for the first and the last time window.
-`order by asc/desc` has no effect on the result.
-`group by tbname` must be used together when `elapsed` is used against a STable.
-`group by` must NOT be used together when `elapsed` is used against a table or sub table.
- When used in nested query, it's only applicable when the inner query outputs an implicit timestamp column as the primary key. For example, `select elapsed(ts) from (select diff(value) from sub1)` is legal usage while `select elapsed(ts) from (select * from sub1)` is not.
- It cannot be used with `leastsquares`, `diff`, `derivative`, `top`, `bottom`, `last_row`, `interp`.
- When used in nested query, it's only applicable when the inner query outputs an implicit timestamp column as the primary key. For example, `select elapsed(ts) from (select diff(value) from sub1)` is legal usage while `select elapsed(ts) from (select * from sub1)` is not. In addition, because elapsed has a strict dependency on the timeline, a statement like `select elapsed(ts) from (select diff(value) from st group by tbname) will return a meaningless result.
- It can't be used with `leastsquares`, `diff`, `derivative`, `top`, `bottom`, `last_row`, `interp`.
### LEASTSQUARES
```
```sql
SELECT LEASTSQUARES(field_name, start_val, step_val) FROM tb_name [WHERE clause];
```
**Description**: The linear regression function of the specified column and the timestamp column (primary key), `start_val` is the initial value and `step_val` is the step value.
**Return value type**: VARCHAR string in the format of "(slope, intercept)".
**Return value type**: A string in the format of "(slope, intercept)"
**Applicable data types**: Numeric
**Applicable data types**: Numeric types.
**Applicable table types**: table only
**Applicable table types**: table only.
### SPREAD
```
```sql
SELECT SPREAD(field_name) FROM { tb_name | stb_name } [WHERE clause];
```
**Description**: The difference between the max and the min value of a specific column.
**Description**: The difference between the max and the min of a specific column
**Return value type**: DOUBLE.
**Return value type**: DOUBLE
**Applicable data types**: Numeric types.
**Applicable data types**: Integers and timestamps
**Applicable table types**: table, STable.
**Applicable table types**: standard tables and supertables
**More explanations**: Can be used on a column of TIMESTAMP type, the result time unit precision is same as the current database in use.
### STDDEV
```
```sql
SELECT STDDEV(field_name) FROM tb_name [WHERE clause];
```
**Description**: Standard deviation of a specific column in a table or STable.
**Description**: Standard deviation of a specific column in a table or STable
**Return value type**: DOUBLE.
**Return value type**: DOUBLE
**Applicable data types**: Numeric types.
**Applicable data types**: Numeric
**Applicable table types**: standard tables and supertables
**Applicable table types**: table, STable.
### SUM
```
```sql
SELECT SUM(field_name) FROM tb_name [WHERE clause];
```
**Description**: The summation of values of a specific column in a table or STable.
**Description**: The sum of a specific column in a table or STable
**Return value type**: DOUBLE or BIGINT
**Return value type**: DOUBLE.
**Applicable data types**: Numeric
**Applicable data types**: Numeric types.
**Applicable table types**: standard tables and supertables
**Applicable table types**: table, STable.
### HYPERLOGLOG
```
```sql
SELECT HYPERLOGLOG(field_name) FROM { tb_name | stb_name } [WHERE clause];
```
**Description**:The cardinal number of a specific column is returned by using hyperloglog algorithm.
**Description**：
The cardinal number of a specific column is returned by using hyperloglog algorithm. The benefit of using hyperloglog algorithm is that the memory usage is under control when the data volume is huge.
However, when the data volume is very small, the result may be not accurate, it's recommented to use `select count(data) from (select unique(col) as data from table)` in this case.
**Return value type**: Integer
**Return value type**: INTEGER.
**Applicable data types**: Numeric
**Applicable data types**: All data types.
**Applicable table types**: standard tables and supertables
**More explanations**: The benefit of using hyperloglog algorithm is that the memory usage is under control when the data volume is huge. However, when the data volume is very small, the result may be not accurate, it's recommented to use `select count(data) from (select unique(col) as data from table)` in this case.
### HISTOGRAM
```
```sql
SELECT HISTOGRAM(field_name，bin_type, bin_description, normalized) FROM tb_name [WHERE clause];
```
**Description**：Returns count of data points in user-specified ranges.
**Return value type**：DOUBLE or BIGINT, depends on normalized parameter settings.
**Return value type** If normalized is set to 1, a DOUBLE is returned; otherwise a BIGINT is returned
**Applicable data type**：Numerical types.
**Applicable data types**: Numeric
**Applicable table types**: table, STable.
**Applicable table types**: table, STable
**Explanations**：
- bin_type: parameter to indicate the bucket type, valid inputs are: "user_input", "linear_bin", "log_bin"。
- bin_description: parameter to describe the rule to generate buckets，can be in the following JSON formats for each bin_type respectively:
- "user_input": "[1, 3, 5, 7]": User specified bin values.
"start" - bin starting point. "width" - bin offset. "count" - number of bins generated. "infinity" - whether to add（-inf, inf）as start/end point in generated set of bins.
The above "linear_bin" descriptor generates a set of bins: [-inf, 0.0, 5.0, 10.0, 15.0, 20.0, +inf].
"start" - bin starting point. "factor" - exponential factor of bin offset. "count" - number of bins generated. "infinity" - whether to add（-inf, inf）as start/end point in generated range of bins.
The above "linear_bin" descriptor generates a set of bins: [-inf, 1.0, 2.0, 4.0, 8.0, 16.0, +inf].
- normalized: setting to 1/0 to turn on/off result normalization. Valid values are 0 or 1.
"infinity" - whether to add（-inf, inf）as start/end point in generated range of bins.
The above "log_bin" descriptor generates a set of bins:[-inf, 1.0, 2.0, 4.0, 8.0, 16.0, +inf].
- normalized: setting to 1/0 to turn on/off result normalization.
### PERCENTILE
```
```sql
SELECT PERCENTILE(field_name, P) FROM { tb_name } [WHERE clause];
```
**Description**: The value whose rank in a specific column matches the specified percentage. If such a value matching the specified percentage doesn't exist in the column, an interpolation value will be returned.
**Return value type**: DOUBLE.
**Return value type**: DOUBLE
**Applicable data types**: Numeric types.
**Applicable column types**: Numeric
**Applicable table types**: table.
**Applicable table types**: table only
**More explanations**: _P_ is in range [0,100], when _P_ is 0, the result is same as using function MIN; when _P_ is 100, the result is same as function MAX.
## Selector Functions
Selector functiosn choose one or more rows in the query result according to the semantics. You can specify to output primary timestamp column and other columns including tbname and tags so that you can easily know which rows the selected values belong to.
## Selection Functions
Selection functions return one or more results depending. You can specify the timestamp column, tbname pseudocolumn, or tag columns to show which rows contain the selected value.
**Description**: The least _k_ values of a specific column in a table or STable. If a value has multiple occurrences in the column but counting all of them in will exceed the upper limit _k_, then a part of them will be returned randomly.
**Return value type**: Same as the column being operated upon.
**Return value type**:Same as the data type of the column being operated upon
**Applicable data types**: Numeric types.
**Applicable data types**: Numeric
**Applicable table types**: table, STable.
**Applicable table types**: standard tables and supertables
**More explanations**:
**More explanation**:
- _k_ must be in range [1,100].
- The timestamp associated with the selected values are returned too.
- Can't be used with `FILL`.
- _k_ must be in range [1,100]
- The timestamp associated with the selected values are returned too
- Can't be used with `FILL`
### FIRST
```
```sql
SELECT FIRST(field_name) FROM { tb_name | stb_name } [WHERE clause];
```
**Description**: The first non-null value of a specific column in a table or STable.
**Description**: The first non-null value of a specific column in a table or STable
**Return value type**: Same as the column being operated upon.
**Return value type**:Same as the data type of the column being operated upon
**Applicable data types**: All data types.
**Applicable data types**: Numeric
**Applicable table types**: table, STable.
**Applicable table types**: standard tables and supertables
**More explanations**:
**More explanation**:
- FIRST(\*) can be used to get the first non-null value of all columns
- NULL will be returned if all the values of the specified column are all NULL
SELECT LAST(field_name) FROM { tb_name | stb_name } [WHERE clause];
```
**Description**: The last non-NULL value of a specific column in a table or STable.
**Description**: The last non-NULL value of a specific column in a table or STable
**Return value type**: Same as the column being operated upon.
**Return value type**:Same as the data type of the column being operated upon
**Applicable data types**: All data types.
**Applicable data types**: Numeric
**Applicable table types**: table, STable.
**Applicable table types**: standard tables and supertables
**More explanations**:
**More explanation**:
- LAST(\*) can be used to get the last non-NULL value of all columns
- If the values of a column in the result set are all NULL, NULL is returned for that column; if all columns in the result are all NULL, no result will be returned.
- When it's used on a STable, if there are multiple values with the timestamp in the result set, one of them will be returned randomly and it's not guaranteed that the same value is returned if the same query is run multiple times.
### LAST_ROW
```
```sql
SELECT LAST_ROW(field_name) FROM { tb_name | stb_name };
```
**Description**: The last row of a table or STable.
**Description**: The last row of a table or STable
**Return value type**: Same as the column being operated upon.
**Return value type**:Same as the data type of the column being operated upon
**Applicable data types**: All data type.
**Applicable data types**: Numeric
**Applicable table types**: table, STable.
**Applicable table types**: standard tables and supertables
**More explanations**:
- When it's used against a STable, multiple rows with the same and largest timestamp may exist, in this case one of them is returned randomly and it's not guaranteed that the result is same if the query is run multiple times.
- Cannot be used with `INTERVAL`.
- When it's used on a STable, if there are multiple values with the timestamp in the result set, one of them will be returned randomly and it's not guaranteed that the same value is returned if the same query is run multiple times.
- Can't be used with `INTERVAL`.
### MAX
```
```sql
SELECT MAX(field_name) FROM { tb_name | stb_name } [WHERE clause];
```
**Description**: The maximum value of a specific column of a table or STable.
**Description**: The maximum value of a specific column of a table or STable
**Return value type**:Same as the data type of the column being operated upon
**Return value type**: Same as the data type of the column being operated upon.
**Applicable data types**: Numeric
**Applicable data types**: Numeric types.
**Applicable table types**: standard tables and supertables
**Applicable table types**: table, STable.
### MIN
```
```sql
SELECT MIN(field_name) FROM {tb_name | stb_name} [WHERE clause];
```
**Description**: The minimum value of a specific column in a table or STable.
**Description**: The minimum value of a specific column in a table or STable
**Return value type**:Same as the data type of the column being operated upon
**Return value type**: Same as the data type of the column being operated upon.
**Applicable data types**: Numeric
**Applicable data types**: Numeric types.
**Applicable table types**: standard tables and supertables
**Applicable table types**: table, STable.
### MODE
```
```sql
SELECT MODE(field_name) FROM tb_name [WHERE clause];
```
**Description**:The value which has the highest frequency of occurrence. NULL is returned if there are multiple values which have highest frequency of occurrence.
**Return value type**:Same as the data type of the column being operated upon.
**Return value type**: Same as the input data
**Applicable data types**: All data types.
**Applicable data types**: Numeric
**Applicable table types**: standard tables and supertables
**More explanations**:Considering the number of returned result set is unpredictable, it's suggested to limit the number of unique values to 100,000, otherwise error will be returned.
**Description**: _k_ sampling values of a specific column. The applicable range of _k_ is [1,1000].
**Return value type**: Same as the column being operated.
**Return value type**: Same as the column being operated plus the associated timestamp
**Applicable data types**: All data types.
**Applicable data types**: Any data type except for tags of STable
**Applicable table types**: table, STable.
**Applicable nested query**: Inner query and Outer query
**Applicable nested query**: Inner query and Outer query.
**Applicable table types**: standard tables and supertables
**More explanations**:
**More explanations**:
This function cannot be used in expression calculation.
- Must be used with `PARTITION BY tbname` when it's used on a STable to force the result on each single timeline
- Arithmetic operation cannot be operated on the result of `SAMPLE` function
- Must be used with `Partition by tbname` when it's used on a STable to force the result on each single timeline.
### TAIL
```
```sql
SELECT TAIL(field_name, k, offset_val) FROM {tb_name | stb_name} [WHERE clause];
```
**Description**: The next _k_ rows are returned after skipping the last `offset_val` rows, NULL values are not ignored. `offset_val` is optional parameter. When it's not specified, the last _k_ rows are returned. When `offset_val` is used, the effect is same as `order by ts desc LIMIT k OFFSET offset_val`.
**Parameter value range**: k: [1,100] offset_val: [0,100].
**Parameter value range**: k: [1,100] offset_val: [0,100]
**Return value type**:Same as the data type of the column being operated upon
**Return value type**: Same as the column being operated upon.
**Applicable data types**: Any data type except for timestamp, i.e. the primary key
**Applicable table types**: standard tables and supertables
**Description**: The greatest _k_ values of a specific column in a table or STable. If a value has multiple occurrences in the column but counting all of them in will exceed the upper limit _k_, then a part of them will be returned randomly.
**Return value type**: Same as the column being operated upon.
**Return value type**:Same as the data type of the column being operated upon
**Applicable data types**: Numeric types.
**Applicable data types**: Numeric
**Applicable table types**: table, STable.
**Applicable table types**: standard tables and supertables
**More explanations**:
**More explanation**:
- _k_ must be in range [1,100].
- The timestamp associated with the selected values are returned too.
- Cannot be used with `FILL`.
- _k_ must be in range [1,100]
- The timestamp associated with the selected values are returned too
- Can't be used with `FILL`
### UNIQUE
```
```sql
SELECT UNIQUE(field_name) FROM {tb_name | stb_name} [WHERE clause];
```
**Description**: The values that occur the first time in the specified column. The effect is similar to `distinct` keyword, but it can also be used to match tags or timestamp.
**Description**: The values that occur the first time in the specified column. The effect is similar to `distinct` keyword, but it can also be used to match tags or timestamp. The first occurrence of a timestamp or tag is used.
**Return value type**: Same as the column or tag being operated upon.
**Return value type**:Same as the data type of the column being operated upon
**Applicable data types**: All data types.
**Applicable column types**: Any data types except for timestamp
**More explanations**:
**Applicable table types**: table, STable
- It can be used against table or STable, but can't be used together with time window, like `interval`, `state_window` or `session_window` .
- Considering the number of result sets is unpredictable, it's suggested to limit the distinct values under 100,000 to control the memory usage, otherwise error will be returned.
## Time-Series Specific Functions
## Time-Series Extensions
TDengine provides a set of time-series specific functions to better meet the requirements in querying time-series data. In general databases, similar functionalities can only be achieved with much more complex syntax and much worse performance. TDengine provides these functionalities in builtin functions so that the burden on user side is minimized.
TDengine includes extensions to standard SQL that are intended specifically for time-series use cases. The functions enabled by these extensions require complex queries to implement in general-purpose databases. By offering them as built-in extensions, TDengine reduces user workload.
SELECT CSUM(field_name) FROM { tb_name | stb_name } [WHERE clause]
```
**Description**: The cumulative sum of each row for a specific column. The number of output rows is same as that of the input rows.
**Return value type**: BIGINT for signed integer input types; UNSIGNED BIGINT for unsigned integer input types; DOUBLE for floating point input types.
**Return value type**: Long integer for integers; Double for floating points. uint64_t for unsigned integers
**Applicable data types**: Numeric types.
**Applicable data types**: Numeric
**Applicable table types**: table, STable.
**Nested query**: It can be used in both the outer query and inner query in a nested query.
**Applicable nested query**: Inner query and Outer query.
**Applicable table types**: standard tables and supertables
**More explanations**:
- Arithmetic operation can't be performed on the result of `csum` function
- Can only be used with aggregate functions This function can be used with supertables and standard tables.
- Must be used with `PARTITION BY tbname` when it's used on a STable to force the result on each single timeline
**More explanations**:
- Arithmetic operation cannot be performed on the result of `csum` function.
- Can only be used with aggregate functions.
-`Partition by tbname` must be used together on a STable to force the result on a single timeline.
### DERIVATIVE
```
```sql
SELECT DERIVATIVE(field_name, time_interval, ignore_negative) FROM tb_name [WHERE clause];
```
**Description**: The derivative of a specific column. The time rage can be specified by parameter `time_interval`, the minimum allowed time range is 1 second (1s); the value of `ignore_negative` can be 0 or 1, 1 means negative values are ignored.
**Return value type**: DOUBLE.
**Applicable data types**: Numeric types.
**Return value type**: DOUBLE
**Applicable table types**: table, STable.
**Applicable data types**: Numeric
**More explanations**:
**Applicable table types**: standard tables and supertables
- The number of result rows is the number of total rows in the time range subtracted by one, no output for the first row.
**More explanation**:
- It can be used together with `PARTITION BY tbname` against a STable.
-Can be used together with selection of relative columns. E.g. select \_rowts, DERIVATIVE() from.
- It can be used together with a selected column. For example: select \_rowts, DERIVATIVE() from。
**Description**: The different of each row with its previous row for a specific column. `ignore_negative` can be specified as 0 or 1, the default value is 1 if it's not specified. `1` means negative values are ignored.
**Return value type**: Same as the column being operated upon.
**Return value type**:Same as the data type of the column being operated upon
**Applicable data types**: Numeric types.
**Applicable data types**: Numeric
**Applicable table types**: table, STable.
**Applicable table types**: standard tables and supertables
**More explanations**:
**More explanation**:
- The number of result rows is the number of rows subtracted by one, no output for the first row
- It can be used together with a selected column. For example: select \_rowts, DIFF() from。
- The number of result rows is the number of rows subtracted by one, no output for the first row.
- It can be used on STable with `PARTITION by tbname`.
- Can be used together with selection of relative columns. E.g. select \_rowts, DIFF() from.
### IRATE
```
```sql
SELECT IRATE(field_name) FROM tb_name WHERE clause;
```
**Description**: instantaneous rate on a specific column. The last two samples in the specified time range are used to calculate instantaneous rate. If the last sample value is smaller, then only the last sample value is used instead of the difference between the last two sample values.
**Return value type**: DOUBLE.
**Return value type**: DOUBLE
**Applicable data types**: Numeric types.
**Applicable data types**: Numeric
**Applicable table types**: table, STable.
**Applicable table types**: standard tables and supertables
**More explanations**:
- It can be used on stble with `PARTITION BY`, i.e. timelines generated by `PARTITION BY tbname` on a STable.
**Description**: The moving average of continuous _k_ values of a specific column. If the number of input rows is less than _k_, nothing is returned. The applicable range of _k_ is [1,1000].
**Return value type**: DOUBLE.
**Return value type**: DOUBLE
**Applicable data types**: Numeric types.
**Applicable data types**: Numeric
**Applicable nested query**: Inner query and Outer query.
**Nested query**: It can be used in both the outer query and inner query in a nested query.
**Applicable table types**: table, STable.
**Applicable table types**: standard tables and supertables
**More explanations**:
**More explanations**:
- Arithmetic operation can't be performed on the result of `MAVG`.
- Can only be used with data columns, can't be used with tags. - Can't be used with aggregate functions.
- Must be used with `PARTITION BY tbname` when it's used on a STable to force the result on each single timeline
- Arithmetic operation cannot be performed on the result of `MAVG`.
- Cannot be used with aggregate functions.
- Must be used with `PARTITION BY tbname` when it's used on a STable to force the result on each single timeline.
**Description**: The number of continuous rows satisfying the specified conditions for a specific column. If the specified condition is evaluated as true, the number is increased by 1; otherwise the number is reset to -1. If the input value is NULL, then the corresponding row is skipped.
**Description**: The number of continuous rows satisfying the specified conditions for a specific column. The result is shown as an extra column for each row. If the specified condition is evaluated as true, the number is increased by 1; otherwise the number is reset to -1. If the input value is NULL, then the corresponding row is skipped.
**Applicable parameter values**:
- oper : Can be one of "LT" (lower than), "GT" (greater than), "LE" (lower than or euqal to), "GE" (greater than or equal to), "NE" (not equal to), "EQ" (equal to).
- val ： Numeric types.
- oper : Can be one of `LT` (lower than), `GT` (greater than), `LE` (lower than or equal to), `GE` (greater than or equal to), `NE` (not equal to), `EQ` (equal to), the value is case insensitive
- val ： Numeric types
**Return value type**: INTEGER.
**Return value type**: Integer
**Applicable data types**: Numeric types.
**Applicable data types**: Numeric
**Applicable table types**: table, STable.
**Applicable nested query**: Outer query only
**Applicable nested query**: Outer query only.
**Applicable table types**: standard tables and supertables
**More explanations**:
- Must be used together with `PARTITION BY tbname` when it's used on a STable to force the result into each single timeline.
- Cannot be used with window operation, like interval/state_window/session_window.
- Must be used together with `PARTITION BY tbname` when it's used on a STable to force the result into each single timeline]
- Can't be used with window operation, like interval/state_window/session_window
**Description**: The length of time range in which all rows satisfy the specified condition for a specific column. The length for the first row that satisfies the condition is 0. Next, if the condition is evaluated as true for a row, the time interval between current row and its previous row is added up to the time range; otherwise the time range length is reset to -1. If the value of the column is NULL, the corresponding row is skipped.
**Description**: The length of time range in which all rows satisfy the specified condition for a specific column. The result is shown as an extra column for each row. The length for the first row that satisfies the condition is 0. Next, if the condition is evaluated as true for a row, the time interval between current row and its previous row is added up to the time range; otherwise the time range length is reset to -1. If the value of the column is NULL, the corresponding row is skipped.
**Applicable parameter values**:
- oper : Can be one of "LT" (lower than), "GT" (greater than), "LE" (lower than or euqal to), "GE" (greater than or equal to), "NE" (not equal to), "EQ" (equal to).
- val : Numeric types.
- unit : The unit of time interval, can be: 1b(nanosecond), 1u(microsecond),1a(millisecond),1s(second),1m(minute),1h(hour),1d(day),1w(week). If not specified, default is same as the current database time precision in use.
- oper : Can be one of `LT` (lower than), `GT` (greater than), `LE` (lower than or equal to), `GE` (greater than or equal to), `NE` (not equal to), `EQ` (equal to), the value is case insensitive
- val ： Numeric types
- unit: The unit of time interval. Enter one of the following options: 1b (nanoseconds), 1u (microseconds), 1a (milliseconds), 1s (seconds), 1m (minutes), 1h (hours), 1d (days), or 1w (weeks) If you do not enter a unit of time, the precision of the current database is used by default.
**Return value type**: INTEGER.
**Return value type**: Integer
**Applicable data types**: Numeric types.
**Applicable data types**: Numeric
**Applicable table types**: table, STable.
**Applicable nested query**: Outer query only
**Applicable nested query**: Outer query only.
**Applicable table types**: standard tables and supertables
**More explanations**:
- Must be used together with `PARTITION BY tbname` when it's used on a STable to force the result into each single timeline.
- Cannot be used with window operation, like interval/state_window/session_window.
- Must be used together with `PARTITION BY tbname` when it's used on a STable to force the result into each single timeline]
- Can't be used with window operation, like interval/state_window/session_window
### TWA
```
```sql
SELECT TWA(field_name) FROM tb_name WHERE clause;
```
**Description**: Time weighted average on a specific column within a time range.
**Description**: Time weighted average on a specific column within a time range
**Return value type**: DOUBLE.
**Return value type**: DOUBLE
**Applicable data types**: Numeric types.
**Applicable data types**: Numeric
**Applicable table types**: table, STable.
**Applicable table types**: standard tables and supertables
**More explanations**:
- Must be used together with `PARTITION BY tbname` to force the result into each single timeline.
- It can be used on stable with `PARTITION BY`, i.e. timelines generated by `PARTITION BY tbname` on a STable.
## System Information Functions
### DATABASE
```
```sql
SELECT DATABASE();
```
**Description**：Return the current database being used. If the user doesn't specify database when logon and doesn't use `USE` SQL command to switch the datbase, this function returns NULL.
**Description**: The current database. If no database is specified upon logging in and no database is specified with `USE` after login, NULL will be returned by `select database()`.
title:Distinguished Query for Time Series Database
sidebar_label:Time-Series Extensions
title:Time-Series Extensions
---
Aggregation by time window is supported in TDengine. For example, in the case where temperature sensors report the temperature every seconds, the average temperature for every 10 minutes can be retrieved by performing a query with a time window.
Window related clauses are used to divide the data set to be queried into subsets and then aggregation is performed across the subsets. There are three kinds of windows: time window, status window, and session window. There are two kinds of time windows: sliding window and flip time/tumbling window.
As a purpose-built database for storing and processing time-series data, TDengine provides time-series-specific extensions to standard SQL.
## Time Window
These extensions include tag-partitioned queries and windowed queries.
The `INTERVAL` clause is used to generate time windows of the same time interval. The `SLIDING` parameter is used to specify the time step for which the time window moves forward. The query is performed on one time window each time, and the time window moves forward with time. When defining a continuous query, both the size of the time window and the step of forward sliding time need to be specified. As shown in the figure blow, [t0s, t0e] ，[t1s , t1e]， [t2s, t2e] are respectively the time ranges of three time windows on which continuous queries are executed. The time step for which time window moves forward is marked by `sliding time`. Query, filter and aggregate operations are executed on each time window respectively. When the time step specified by `SLIDING` is same as the time interval specified by `INTERVAL`, the sliding time window is actually a flip time/tumbling window.
## Tag-Partitioned Queries
`INTERVAL` and `SLIDING` should be used with aggregate functions and select functions. The SQL statement below is illegal because no aggregate or selection function is used with `INTERVAL`.
When you query a supertable, you may need to partition the supertable by tag and perform additional operations on a specific partition. In this case, you can use the following SQL clause:
```sql
PARTITIONBYpart_list
```
SELECT * FROM temp_tb_1 INTERVAL(1m);
```
The time step specified by `SLIDING` cannot exceed the time interval specified by `INTERVAL`. The SQL statement below is illegal because the time length specified by `SLIDING` exceeds that specified by `INTERVAL`.
```
SELECT COUNT(*) FROM temp_tb_1 INTERVAL(1m) SLIDING(2m);
```
When the time length specified by `SLIDING` is the same as that specified by `INTERVAL`, the sliding window is actually a flip/tumbling window. The minimum time range specified by `INTERVAL` is 10 milliseconds (10a) prior to version 2.1.5.0. Since version 2.1.5.0, the minimum time range by `INTERVAL` can be 1 microsecond (1u). However, if the DB precision is millisecond, the minimum time range is 1 millisecond (1a). Please note that the `timezone` parameter should be configured to be the same value in the `taos.cfg` configuration file on client side and server side.
## Status Window
In case of using integer, bool, or string to represent the status of a device at any given moment, continuous rows with the same status belong to a status window. Once the status changes, the status window closes. As shown in the following figure, there are two status windows according to status, [2019-04-28 14:22:07，2019-04-28 14:22:10] and [2019-04-28 14:22:11，2019-04-28 14:22:12]. Status window is not applicable to STable for now.
part_list can be any scalar expression, such as a column, constant, scalar function, or a combination of the preceding items.
`STATE_WINDOW` is used to specify the column on which the status window will be based. For example：
A PARTITION BY clause with a tag is processed as follows:
```
SELECT COUNT(*), FIRST(ts), status FROM temp_tb_1 STATE_WINDOW(status);
```
## Session Window
- The PARTITION BY clause must occur after the WHERE clause and cannot be used with a JOIN clause.
- The PARTITION BY clause partitions the super table by the specified tag group, and the specified calculation is performed on each partition. The calculation performed is determined by the rest of the statement - a window clause, GROUP BY clause, or SELECT clause.
- You can use PARTITION BY together with a window clause or GROUP BY clause. In this case, the window or GROUP BY clause takes effect on every partition. For example, the following statement partitions the table by the location tag, performs downsampling over a 10 minute window, and returns the maximum value:
The primary key, i.e. timestamp, is used to determine which session window a row belongs to. If the time interval between two adjacent rows is within the time range specified by `tol_val`, they belong to the same session window; otherwise they belong to two different session windows. As shown in the figure below, if the limit of time interval for the session window is specified as 12 seconds, then the 6 rows in the figure constitutes 2 time windows, [2019-04-28 14:22:10，2019-04-28 14:22:30] and [2019-04-28 14:23:10，2019-04-28 14:23:30], because the time difference between 2019-04-28 14:22:30 and 2019-04-28 14:23:10 is 40 seconds, which exceeds the time interval limit of 12 seconds.
If the time interval between two continuous rows are within the time interval specified by `tol_value` they belong to the same session window; otherwise a new session window is started automatically. Session window is not supported on STable for now.
## More On Window Aggregate
### Syntax
## Windowed Queries
The full syntax of aggregate by window is as follows:
Aggregation by time window is supported in TDengine. For example, in the case where temperature sensors report the temperature every seconds, the average temperature for every 10 minutes can be retrieved by performing a query with a time window. Window related clauses are used to divide the data set to be queried into subsets and then aggregation is performed across the subsets. There are three kinds of windows: time window, status window, and session window. There are two kinds of time windows: sliding window and flip time/tumbling window. The query syntax is as follows:
```sql
SELECTfunction_listFROMtb_name
...
...
@@ -63,27 +38,36 @@ SELECT function_list FROM tb_name
[STATE_WINDOW(col)]
[INTERVAL(interval[,offset])[SLIDINGsliding]]
[FILL({NONE|VALUE|PREV|NULL|LINEAR|NEXT})]
SELECTfunction_listFROMstb_name
[WHEREwhere_condition]
[INTERVAL(interval[,offset])[SLIDINGsliding]]
[FILL({NONE|VALUE|PREV|NULL|LINEAR|NEXT})]
[GROUPBYtags]
```
### Restrictions
The following restrictions apply:
### Restricted Functions
- Aggregate functions and select functions can be used in `function_list`, with each function having only one output. For example COUNT, AVG, SUM, STDDEV, LEASTSQUARES, PERCENTILE, MIN, MAX, FIRST, LAST. Functions having multiple outputs, such as DIFF or arithmetic operations can't be used.
-`LAST_ROW` can't be used together with window aggregate.
- Scalar functions, like CEIL/FLOOR, can't be used with window aggregate.
### Other Rules
- The window clause must occur after the PARTITION BY clause and before the GROUP BY clause. It cannot be used with a GROUP BY clause.
- SELECT clauses on windows can contain only the following expressions:
- Constants
- Aggregate functions
- Expressions that include the preceding expressions.
- The window clause cannot be used with a GROUP BY clause.
-`WHERE` clause can be used to specify the starting and ending time and other filter conditions
-`FILL` clause is used to specify how to fill when there is data missing in any window, including:
1. NONE: No fill (the default fill mode)
2. VALUE：Fill with a fixed value, which should be specified together, for example `FILL(VALUE, 1.23)`
3. PREV：Fill with the previous non-NULL value, `FILL(PREV)`
4. NULL：Fill with NULL, `FILL(NULL)`
5. LINEAR：Fill with the closest non-NULL value, `FILL(LINEAR)`
6. NEXT：Fill with the next non-NULL value, `FILL(NEXT)`
### FILL Clause
`FILL` clause is used to specify how to fill when there is data missing in any window, including:
1. NONE: No fill (the default fill mode)
2. VALUE：Fill with a fixed value, which should be specified together, for example `FILL(VALUE, 1.23)` Note: The value filled depends on the data type. For example, if you run FILL(VALUE 1.23) on an integer column, the value 1 is filled.
3. PREV：Fill with the previous non-NULL value, `FILL(PREV)`
4. NULL：Fill with NULL, `FILL(NULL)`
5. LINEAR：Fill with the closest non-NULL value, `FILL(LINEAR)`
6. NEXT：Fill with the next non-NULL value, `FILL(NEXT)`
:::info
...
...
@@ -93,17 +77,66 @@ SELECT function_list FROM stb_name
:::
Aggregate by time window is also used in continuous query, please refer to [Continuous Query](/develop/continuous-query).
### Time Window
## Examples
There are two kinds of time windows: sliding window and flip time/tumbling window.
The `INTERVAL` clause is used to generate time windows of the same time interval. The `SLIDING` parameter is used to specify the time step for which the time window moves forward. The query is performed on one time window each time, and the time window moves forward with time. When defining a continuous query, both the size of the time window and the step of forward sliding time need to be specified. As shown in the figure blow, [t0s, t0e] ，[t1s , t1e]， [t2s, t2e] are respectively the time ranges of three time windows on which continuous queries are executed. The time step for which time window moves forward is marked by `sliding time`. Query, filter and aggregate operations are executed on each time window respectively. When the time step specified by `SLIDING` is same as the time interval specified by `INTERVAL`, the sliding time window is actually a flip time/tumbling window.
`INTERVAL` and `SLIDING` should be used with aggregate functions and select functions. The SQL statement below is illegal because no aggregate or selection function is used with `INTERVAL`.
```
SELECT * FROM temp_tb_1 INTERVAL(1m);
```
The time step specified by `SLIDING` cannot exceed the time interval specified by `INTERVAL`. The SQL statement below is illegal because the time length specified by `SLIDING` exceeds that specified by `INTERVAL`.
```
SELECT COUNT(*) FROM temp_tb_1 INTERVAL(1m) SLIDING(2m);
```
When using time windows, note the following:
- The window length for aggregation depends on the value of INTERVAL. The minimum interval is 10 ms. You can configure a window as an offset from UTC 0:00. The offset cannot be smaler than the interval. You can use SLIDING to specify the length of time that the window moves forward.
Please note that the `timezone` parameter should be configured to be the same value in the `taos.cfg` configuration file on client side and server side.
- The result set is in ascending order of timestamp when you aggregate by time window.
### Status Window
In case of using integer, bool, or string to represent the status of a device at any given moment, continuous rows with the same status belong to a status window. Once the status changes, the status window closes. As shown in the following figure, there are two status windows according to status, [2019-04-28 14:22:07，2019-04-28 14:22:10] and [2019-04-28 14:22:11，2019-04-28 14:22:12]. Status window is not applicable to STable for now.
`STATE_WINDOW` is used to specify the column on which the status window will be based. For example:
```
SELECT COUNT(*), FIRST(ts), status FROM temp_tb_1 STATE_WINDOW(status);
```
### Session Window
The primary key, i.e. timestamp, is used to determine which session window a row belongs to. As shown in the figure below, if the limit of time interval for the session window is specified as 12 seconds, then the 6 rows in the figure constitutes 2 time windows, [2019-04-28 14:22:10，2019-04-28 14:22:30] and [2019-04-28 14:23:10，2019-04-28 14:23:30] because the time difference between 2019-04-28 14:22:30 and 2019-04-28 14:23:10 is 40 seconds, which exceeds the time interval limit of 12 seconds.
If the time interval between two continuous rows are within the time interval specified by `tol_value` they belong to the same session window; otherwise a new session window is started automatically. Session window is not supported on STable for now.
```
SELECT COUNT(*), FIRST(ts) FROM temp_tb_1 SESSION(ts, tol_val);
```
### Examples
A table of intelligent meters can be created by the SQL statement below：
```sql
```
CREATE TABLE meters (ts TIMESTAMP, current FLOAT, voltage INT, phase FLOAT) TAGS (location BINARY(64), groupId INT);
```
The average current, maximum current and median of current in every 10 minutes for the past 24 hours can be calculated using the SQL statement below, with missing values filled with the previous non-NULL values.
The average current, maximum current and median of current in every 10 minutes for the past 24 hours can be calculated using the SQL statement below, with missing values filled with the previous non-NULL values. The query statement is as follows:
```
SELECT AVG(current), MAX(current), APERCENTILE(current, 50) FROM meters
You can use filtering, scalar functions, and user-defined scalar functions with a topic. JOIN, GROUP BY, windows, aggregate functions, and user-defined aggregate functions are not supported. The following rules apply to subscribing to a column:
1. The returned field is determined when the topic is created.
2.Columns to which a consumer is subscribed or that are involved in calculations cannot be deleted or modified.
3.If you add a column, the new column will not appear in the results for the subscription.
4.If you run `SELECT \*`, all columns in the subscription at the time of its creation are displayed. This includes columns in supertables, standard tables, and subtables. Supertables are shown as data columns plus tag columns.
**超级表订阅和数据库订阅**规则如下：
1. 被订阅主体的 schema 变更不受限
2. 返回消息中 schema 是块级别的，每块的 schema 可能不一样
3. 列变更后写入的数据若未落盘，将以写入时的 schema 返回
4. 列变更后写入的数据若未已落盘，将以落盘时的 schema 返回
## 删除订阅主题
## Delete a Topic
```sql
DROPTOPIC[IFEXISTS]topic_name;
```
此时如果该订阅主题上存在 consumer，则此 consumer 会收到一个错误。
If a consumer is subscribed to the topic that you delete, the consumer will receive an error.
## 查看订阅主题
## View Topics
## SHOW TOPICS
...
...
@@ -43,24 +36,24 @@ DROP TOPIC [IF EXISTS] topic_name;
SHOWTOPICS;
```
显示当前数据库下的所有主题的信息。
The preceding command displays all topics in the current database.
## 创建消费组
## Create Consumer Group
消费组的创建只能通过 TDengine 客户端驱动或者连接器所提供的 API 创建。
You can create consumer groups only through the TDengine Client driver or the API provided by a connector.
Raw time-series data is often cleaned and preprocessed before being permanently stored in a database. Stream processing components like Kafka, Flink, and Spark are often deployed alongside a time-series database to handle these operations, increasing system complexity and maintenance costs.
Because stream processing is built in to TDengine, you are no longer reliant on middleware. TDengine offers a unified platform for writing, preprocessing, permanent storage, complex analysis, and real-time computation and alerting. Additionally, you can use SQL to perform all these tasks.
Session windows, state windows, and sliding windows are supported. When you configure a session or state window for a supertable, you must use PARTITION BY TBNAME.
`SESSION` indicates a session window, and `tol_val` indicates the maximum range of the time interval. If the time interval between two continuous rows are within the time interval specified by `tol_val` they belong to the same session window; otherwise a new session window is started automatically.
For example, the following SQL statement creates a stream and automatically creates a supertable named `avg_vol`. The stream has a 1 minute time window that slides forward in 30 second intervals to calculate the average voltage of the meters supertable.
When you create a stream, you can use the TRIGGER parameter to specify triggering conditions for it.
MAX_DELAY 模式在窗口关闭时会立即触发计算。此外，当数据写入后，计算触发的时间超过 max delay 指定的时间，则立即触发计算
For non-windowed processing, triggering occurs in real time. For windowed processing, there are three methods of triggering:
## 流式计算的乱序数据容忍策略
1. AT_ONCE: triggers on write
在创建流时，可以在 stream_option 中指定 watermark。
2. WINDOW_CLOSE: triggers when the window closes. This is determined by the event time. You can use WINDOW_CLOSE together with `watermark`. For more information, see Stream Processing Strategy for Out-of-Order Data.
流式计算通过 watermark 来度量对乱序数据的容忍程度，watermark 默认为 0。
3. MAX_DELAY: triggers when the window closes. If the window has not closed but the time elapsed exceeds MAX_DELAY, stream processing is also triggered.
T = 最新事件时间 - watermark
Because the window closing is determined by the event time, a delay or termination of an event stream will prevent the event time from being updated. This may result in an inability to obtain the latest results.
每批到来的数据都会以上述公式更新窗口关闭时间，并将窗口结束时间 < T 的所有打开的窗口关闭，若触发模式为 WINDOW_CLOSE 或 MAX_DELAY，则推送窗口聚合结果。
For this reason, MAX_DELAY is provided as a way to ensure that processing occurs even if the window does not close.
流式计算的过期数据处理策略
对于已关闭的窗口，再次落入该窗口中的数据被标记为过期数据，对于过期数据，流式计算提供两种处理方式：
MAX_DELAY also triggers when the window closes. Additionally, if a write occurs but the processing is not triggered before MAX_DELAY expires, processing is also triggered.
1. 直接丢弃：这是常见流式计算引擎提供的默认（甚至是唯一）计算模式
## Stream Processing Strategy for Out-of-Order Data
2. 重新计算：从 TSDB 中重新查找对应窗口的所有数据并重新计算得到最新结果
When you create a stream, you can specify a watermark in the `stream_option` parameter.
The watermark is used to specify the tolerance for out-of-order data. The default value is 0.
## 流式计算的数据填充策略
T = latest event time - watermark
TODO
The window closing time for each batch of data that arrives at the system is updated using the preceding formula, and all windows are closed whose closing time is less than T. If the triggering method is WINDOW_CLOSE or MAX_DELAY, the aggregate result for the window is pushed.
## 流式计算与会话窗口（session window）
Stream processing strategy for expired data
The data in expired windows is tagged as expired. TDengine stream processing provides two methods for handling such data:
1. Drop the data. This is the default and often only handling method for most stream processing engines.
## 流式计算的监控与流任务分布查询
2. Recalculate the data. In this method, all data in the window is reobtained from the database and recalculated. The latest results are then returned.
TODO
## 流式计算的内存控制与存算分离
TODO
## 流式计算的暂停与恢复
```sql
STOPSTREAMstream_name;
RESUMESTREAMstream_name;
```
In both of these methods, configuring the watermark is essential for obtaining accurate results (if expired data is dropped) and avoiding repeated triggers that affect system performance (if expired data is recalculated).
`->` operator can be used to get the value of a key in a column of JSON type, the left oeprand is the column name, the right operand is a string constant. For example, `col->'name'` returns the value of key `'name'`.
The `->` operator returns the value for a key in JSON column. Specify the column indicator on the left of the operator and the key name on the right of the operator. For example, `col->name` returns the value of the name key.
## Set Operator
## Set Operators
Set operators are used to combine the results of two queries into single result. A query including set operators is called a combined query. The number of rows in each result in a combined query must be same, and the type is determined by the first query's result, the type of the following queriess result must be able to be converted to the type of the first query's result, the conversion rule is same as `CAST` function.
Set operators combine the results of two queries. Queries that include set operators are known as compound queries. The expressions corresponding to each query in the select list in a compound query must match in number. The results returned take the data type of the first query, and the data type returned by subsequent queries must be convertible into the data type of the first query. The conditions of the `CAST` function apply to this conversion.
TDengine provides 2 set operators: `UNION ALL` and `UNION`. `UNION ALL` combines the results without removing duplicate data. `UNION` combines the results and remove duplicate data rows. In single SQL statement, at most 100 set operators can be used.
TDengine supports the `UNION` and `UNION ALL` operations. UNION ALL collects all query results and returns them as a composite result without deduplication. UNION collects all query results and returns them as a deduplicated composite result. In a single SQL statement, at most 100 set operators can be supported.
`LIKE` operator uses wildcard to match a string, the rules are：
LIKE is used together with wildcards to match strings. Its usage is described as follows:
- '%' matches 0 to any number of characters; '\_' matches any single ASCII character.
-\_ can be used to match a `_` in the string, i.e. using escape character backslash `\`
- Wildcard string is 100 bytes at most. Longer a wildcard string is, worse the performance of LIKE operator is.
- '%' matches 0 or any number of characters, '\_' matches any single ASCII character.
-`\_` is used to match the \_ in the string.
-The maximum length of wildcard string is 100 bytes. A very long wildcard string may slowdown the execution performance of `LIKE` operator.
`MATCH` and `NMATCH` operators use regular expressions to match a string, the rules are:
MATCH and NMATCH are used together with regular expressions to match strings. Their usage is described as follows:
- Regular expressions of POSIX standard are supported.
- Only `tbname`, i.e. table name of sub tables, and tag columns of string types can be matched with regular expression, data columns are not supported.
- Regular expression string is 128 bytes at most, and can be adjusted by setting parameter `maxRegexStringLen`, which is a client side configuration and needs to restart the client to take effect.
-Use POSIX regular expression syntax. For more information, see Regular Expressions.
-Regular expression can be used against only table names, i.e. `tbname`, and tags of binary/nchar types, but can't be used against data columns.
-The maximum length of regular expression string is 128 bytes. Configuration parameter `maxRegexStringLen` can be used to set the maximum allowed regular expression. It's a configuration parameter on the client side, and will take effect after restarting the client.
| 1 | AND | BOOL | Logical AND; if both conditions are true, TRUE is returned; If either condition is false, FALSE is returned.
| 2 | OR | BOOL | Logical OR; if either condition is true, TRUE is returned; If both conditions are false, FALSE is returned.
TDengine uses shortcircut optimization when performing logical operations. For AND operator, if the first condition is evaluated to FALSE, then the second one is not evaluated. For OR operator, if the first condition is evaluated to TRUE, then the second one is not evaluated.
TDengine performs short-path optimization when calculating logical conditions. If the first condition for AND is false, FALSE is returned without calculating the second condition. If the first condition for OR is true, TRUE is returned without calculating the second condition
1.Names can include letters, digits, and underscores (_).
2.Names can begin with letters or underscores (_) but not with digits.
3.Names are not case-sensitive.
4.Rules for names with escape characters are as follows:
You can escape a name by enclosing it in backticks (`). In this way, you can reuse keyword names for table names. However, the first three naming rules no longer apply.
Table and column names that are enclosed in escape characters are still subject to length limits. When the length of such a name is calculated, the escape characters are not included. Names specified using escape character are case-sensitive.
For example, \`aBc\` and \`abc\` are different table or column names, but "abc" and "aBc" are same names because internally they are all "abc".
Only ASCII visible characters can be used with escape character.
## 密码合法字符集
## Password Rules
`[a-zA-Z0-9!?$%^&*()_–+={[}]:;@~#|<,>.?/]`
去掉了 `` ‘“`\ `` (单双引号、撇号、反斜杠、空格)
The following characters cannot occur in a password: single quotation marks ('), double quotation marks ("), backticks (`), backslashes (\\), and spaces.
-Maximum length of table name is 192 bytes, excluding the database name prefix and the separator.
-Maximum length of each data row is 48K bytes. Note that the upper limit includes the extra 2 bytes consumed by each column of BINARY/NCHAR type.
-The maximum length of a column name is 64 bytes.
-Maximum number of columns is 4096. There must be at least 2 columns, and the first column must be timestamp.
-The maximum length of a tag name is 64 bytes
-Maximum number of tags is 128. There must be at least 1 tag. The total length of tag values cannot exceed 16 KB.
-Maximum length of single SQL statement is 1 MB (1048576 bytes). It can be configured in the parameter `maxSQLLength` in the client side, the applicable range is [65480, 1048576].
-At most 4096 columns can be returned by `SELECT`. Functions in the query statement constitute columns. An error is returned if the limit is exceeded.
-Maximum numbers of databases, STables, tables are dependent only on the system resources.
-The number of replicas can only be 1 or 3.
-The maximum length of a username is 23 bytes.
-The maximum length of a password is 15 bytes.
-The maximum number of rows depends on system resources.
-The maximum number of vnodes in a database is 1024.
The name of a table or column can only be composed of ASCII characters, digits and underscore and it cannot start with a digit. The maximum length is 192 bytes. Names are case insensitive. The name mentioned in this rule doesn't include the database name prefix and the separator.
To support more flexible table or column names, new escape character "\`" is introduced in TDengine to avoid the conflict between table name and keywords and break the above restrictions for table names. The escape character is not counted in the length of table name.
With escaping, the string inside escape characters are case sensitive, i.e. will not be converted to lower case internally. The table names specified using escape character are case sensitive.
There are about 200 keywords reserved by TDengine, they can't be used as the name of database, STable or table with either upper case, lower case or mixed case.
## Keyword List
## Keywords List
There are about 200 keywords reserved by TDengine, they can't be used as the name of database, STable or table with either upper case, lower case or mixed case. The following list shows all reserved keywords:
### A
...
...
@@ -57,70 +58,70 @@ There are about 200 keywords reserved by TDengine, they can't be used as the nam
### D
- DATABASE
- DATABASES
- DAYS
- DBS
- DEFERRED
- DATABASE
- DATABASES
- DAYS
- DBS
- DEFERRED
- DELETE
- DELIMITERS
- DESC
- DESCRIBE
- DETACH
- DISTINCT
- DIVIDE
- DNODE
- DNODES
- DOT
- DOUBLE
- DROP
- DESC
- DESCRIBE
- DETACH
- DISTINCT
- DIVIDE
- DNODE
- DNODES
- DOT
- DOUBLE
- DROP
### E
- END
- EQ
- EXISTS
- EXPLAIN
- END
- EQ
- EXISTS
- EXPLAIN
### F
- FAIL
- FILE
- FILL
- FLOAT
- FOR
- FROM
- FSYNC
- FAIL
- FILE
- FILL
- FLOAT
- FOR
- FROM
- FSYNC
### G
- GE
- GLOB
- GE
- GLOB
- GRANTS
- GROUP
- GT
- GROUP
- GT
### H
- HAVING
- HAVING
### I
- ID
- IF
- IGNORE
- IGNORE
- IMMEDIA
- IMPORT
- IN
- IMPORT
- IN
- INITIAL
- INSERT
- INSERT
- INSTEAD
- INT
- INT
- INTEGER
- INTERVA
- INTO
- IS
- ISNULL
- INTO
- IS
- ISNULL
### J
...
...
@@ -129,187 +130,147 @@ There are about 200 keywords reserved by TDengine, they can't be used as the nam
### K
- KEEP
- KEY
- KEY
- KILL
### L
- LE
- LIKE
- LIMIT
- LE
- LIKE
- LIMIT
- LINEAR
- LOCAL
- LP
- LOCAL
- LP
- LSHIFT
- LT
- LT
### M
- MATCH
- MAXROWS
- MINROWS
- MINUS
- MNODES
- MODIFY
- MODULES
- MATCH
- MAXROWS
- MINROWS
- MINUS
- MNODES
- MODIFY
- MODULES
### N
- NE
- NONE
- NOT
- NE
- NONE
- NOT
- NOTNULL
- NOW
- NOW
- NULL
### O
- OF
- OF
- OFFSET
- OR
- ORDER
- OR
- ORDER
### P
- PARTITION
- PASS
- PLUS
- PPS
- PASS
- PLUS
- PPS
- PRECISION
- PREV
- PREV
- PRIVILEGE
### Q
- QTIME
- QTIME
- QUERIE
- QUERY
- QUERY
- QUORUM
### R
- RAISE
- REM
- RAISE
- REM
- REPLACE
- REPLICA
- RESET
- RESET
- RESTRIC
- ROW
- RP
- ROW
- RP
- RSHIFT
### S
- SCORES
- SELECT
- SEMI
- SCORES
- SELECT
- SEMI
- SESSION
- SET
- SHOW
- SLASH
- SET
- SHOW
- SLASH
- SLIDING
- SLIMIT
- SLIMIT
- SMALLIN
- SOFFSET
- STable
- STable
- STableS
- STAR
- STATE
- STAR
- STATE
- STATEMEN
- STATE_WI
- STORAGE
- STREAM
- STREAMS
- STRING
- SYNCDB
- STORAGE
- STREAM
- STREAMS
- STRING
- SYNCDB
### T
- TABLE
- TABLES
- TAG
- TAGS
- TBNAME
- TIMES
- TIMESTAMP
- TINYINT
- TOPIC
- TOPICS
- TRIGGER
- TSERIES
- TABLE
- TABLES
- TAG
- TAGS
- TBNAME
- TIMES
- TIMESTAMP
- TINYINT
- TOPIC
- TOPICS
- TRIGGER
- TSERIES
### U
- UMINUS
- UNION
- UNSIGNED
- UPDATE
- UPLUS
- USE
- USER
- USERS
- USING
- UMINUS
- UNION
- UNSIGNED
- UPDATE
- UPLUS
- USE
- USER
- USERS
- USING
### V
- VALUES
- VARIABLE
- VALUES
- VARIABLE
- VARIABLES
- VGROUPS
- VIEW
- VNODES
- VGROUPS
- VIEW
- VNODES
### W
- WAL
- WHERE
### _
- _C0
- _QSTART
- _QSTOP
- _QDURATION
- _WSTART
- _WSTOP
- _WDURATION
## Explanations
### TBNAME
`TBNAME` can be considered as a special tag, which represents the name of the subtable, in a STable.
Get the table name and tag values of all subtables in a STable.
```mysql
SELECT TBNAME, location FROM meters;
```
Count the number of subtables in a STable.
```mysql
SELECT COUNT(TBNAME) FROM meters;
```
Only filter on TAGS can be used in WHERE clause in the above two query statements.
The physical entities that form TDengine clusters are known as data nodes (dnodes). Each dnode is a process running on the operating system of the physical machine. Dnodes can contain virtual nodes (vnodes), which store time-series data. Virtual nodes are formed into vgroups, which have 1 or 3 vnodes depending on the replica setting. If you want to enable replication on your cluster, it must contain at least three nodes. Dnodes can also contain management nodes (mnodes). Each cluster has up to three mnodes. Finally, dnodes can contain query nodes (qnodes), which compute time-series data, thus separating compute from storage. A single dnode can contain a vnode, qnode, and mnode.
Create the dnode before starting the corresponding dnode process. The dnode can then join the cluster based on the value of the firstEp parameter. Each dnode is assigned an ID after it joins a cluster.
You can delete a dnode by its ID or by its endpoint. Note that deleting a dnode does not stop its process. You must stop the process after the dnode is deleted.
The parameters that you can modify through this statement are the same as those located in the dnode configuration file. Modifications that you make through this statement take effect immediately, while modifications to the configuration file take effect when the dnode restarts.
TDengine automatically creates an mnode on the firstEp node. You can use this statement to create more mnodes for higher system availability. A cluster can have a maximum of three mnodes. Each dnode can contain only one mnode.
## 查看管理节点
## View Mnodes
```sql
SHOWMNODES;
```
列出集群中所有的管理节点，包括其 ID，所在 DNODE 以及状态。
This statement shows all mnodes in the cluster with the ID, dnode, and status.
## 删除管理节点
## Delete an Mnode
```sql
DROPMNODEONDNODEdnode_id;
```
删除 dnode_id 所指定的 DNODE 上的 MNODE。
This statement deletes the mnode located on the specified dnode.
TDengine does not automatically create qnodes on startup. You can create qnodes as necessary for compute/storage separation. Each dnode can contain only one qnode. If a qnode is created on a dnode whose supportVnodes parameter is not 0, a vnode and qnode may coexist on the dnode. Each dnode can have a maximum of one vnode, one qnode, and one mnode. However, you can configure your cluster so that vnodes, qnodes, and mnodes are located on separate dnodes. If you set supportVnodes to 0 for a dnode, you can then decide whether to deploy an mnode or a qnode on it. In this way you can physically separate virtual node types.
## 查看查询节点
## View Qnodes
```sql
SHOWQNODES;
```
列出集群中所有查询节点，包括 ID，及所在 DNODE。
This statement shows all qnodes in the cluster with the ID and dnode.
## 删除查询节点
## Delete a Qnode
```sql
DROPQNODEONDNODEdnode_id;
```
删除 ID 为 dnode_id 的 DNODE 上的 QNODE，但并不会影响该 dnode 的状态。
This statement deletes the mnode located on the specified dnode. This does not affect the status of the dnode.
## 修改客户端配置
## Modify Client Configuration
如果将客户端也看作广义的集群的一部分，可以通过如下命令动态修改客户端配置参数。
The client configuration can also be modified in a similar way to other cluster components.
```sql
ALTERLOCALlocal_option
...
...
@@ -129,26 +129,26 @@ local_option: {
}
```
上面语法中的参数与在配置文件中配置客户端的用法相同，但不需要重启客户端，修改后立即生效。
The parameters that you can modify through this statement are the same as those located in the client configuration file. Modifications that you make through this statement take effect immediately, while modifications to the configuration file take effect when the client restarts.
If load and data are not properly balanced among vgroups due to the data in different tim lines having different characteristics, you can combine or separate vgroups.
This statement creates a new vgroup and migrates part of the data from the original vgroup to the new vgroup with consistent hashing. During this process, the original vgroup can continue to provide services normally.
TDengine 内置了一个名为 `INFORMATION_SCHEMA` 的数据库，提供对数据库元数据、数据库系统信息和状态的访问，例如数据库或表的名称，当前执行的 SQL 语句等。该数据库存储有关 TDengine 维护的所有其他数据库的信息。它包含多个只读表。实际上，这些表都是视图，而不是基表，因此没有与它们关联的文件。所以对这些表只能查询，不能进行 INSERT 等写入操作。`INFORMATION_SCHEMA` 数据库旨在以一种更一致的方式来提供对 TDengine 支持的各种 SHOW 语句（如 SHOW TABLES、SHOW DATABASES）所提供的信息的访问。与 SHOW 语句相比，使用 SELECT ... FROM INFORMATION_SCHEMA.tablename 具有以下优点：
TDengine includes a built-in database named `INFORMATION_SCHEMA` to provide access to database metadata, system information, and status information. This information includes database names, table names, and currently running SQL statements. All information related to TDengine maintenance is stored in this database. It contains several read-only tables. These tables are more accurately described as views, and they do not correspond to specific files. You can query these tables but cannot write data to them. The INFORMATION_SCHEMA database is intended to provide a unified method for SHOW commands to access data. However, using SELECT ... FROM INFORMATION_SCHEMA.tablename offers several advantages over SHOW commands:
1.You can use a USE statement to specify the INFORMATION_SCHEMA database as the current database.
2.You can use the familiar SELECT syntax to access information, provided that you know the table and column names.
3.You can filter and order the query results. More generally, you can use any SELECT syntax that TDengine supports to query the INFORMATION_SCHEMA database.
4.Future versions of TDengine can add new columns to INFORMATION_SCHEMA tables without affecting existing business systems.
5.It is easier for users coming from other database management systems. For example, Oracle users can query data dictionary tables.
Note: 由于 SHOW 语句已经被开发者熟悉和广泛使用，所以它们仍然被保留。
Note: SHOW statements are still supported for the convenience of existing users.
本章将详细介绍 `INFORMATION_SCHEMA` 这个内置元数据库中的表和表结构。
This document introduces the tables of INFORMATION_SCHEMA and their structure.
## INS_DNODES
提供 dnode 的相关信息。也可以使用 SHOW DNODES 来查询这些信息。
Provides information about dnodes. Similar to SHOW DNODES.
TDengine includes a built-in database named `PERFORMANCE_SCHEMA` to provide access to database performance statistics. This document introduces the tables of PERFORMANCE_SCHEMA and their structure.
## PERF_APP
Provides information about clients (such as applications) that connect to the cluster. Similar to SHOW APPS.
In addition to running SELECT statements on INFORMATION_SCHEMA, you can also use SHOW to obtain system metadata, information, and status.
## SHOW ACCOUNTS
...
...
@@ -11,9 +11,9 @@ title: 使用 SHOW 命令查看系统元数据
SHOWACCOUNTS;
```
显示当前系统中所有租户的信息。
Shows information about tenants on the system.
注：企业版独有
Note: TDengine Enterprise Edition only.
## SHOW APPS
...
...
@@ -21,7 +21,7 @@ SHOW ACCOUNTS;
SHOWAPPS;
```
显示接入集群的应用（客户端）信息。
Shows all clients (such as applications) that connect to the cluster.
## SHOW BNODES
...
...
@@ -29,7 +29,7 @@ SHOW APPS;
SHOWBNODES;
```
显示当前系统中存在的 BNODE (backup node, 即备份节点）的信息。
Shows information about backup nodes (bnodes) in the system.
## SHOW CLUSTER
...
...
@@ -37,7 +37,7 @@ SHOW BNODES;
SHOWCLUSTER;
```
显示当前集群的信息
Shows information about the current cluster.
## SHOW CONNECTIONS
...
...
@@ -45,7 +45,7 @@ SHOW CLUSTER;
SHOWCONNECTIONS;
```
显示当前系统中存在的连接的信息。
Shows information about connections to the system.
## SHOW CONSUMERS
...
...
@@ -53,7 +53,7 @@ SHOW CONNECTIONS;
SHOWCONSUMERS;
```
显示当前数据库下所有活跃的消费者的信息。
Shows information about all active consumers in the system.
## SHOW CREATE DATABASE
...
...
@@ -61,7 +61,7 @@ SHOW CONSUMERS;
SHOWCREATEDATABASEdb_name;
```
显示 db_name 指定的数据库的创建语句。
Shows the SQL statement used to create the specified database.
## SHOW CREATE STABLE
...
...
@@ -69,7 +69,7 @@ SHOW CREATE DATABASE db_name;
SHOWCREATESTABLE[db_name.]stb_name;
```
显示 tb_name 指定的超级表的创建语句
Shows the SQL statement used to create the specified supertable.
## SHOW CREATE TABLE
...
...
@@ -77,7 +77,7 @@ SHOW CREATE STABLE [db_name.]stb_name;
SHOWCREATETABLE[db_name.]tb_name
```
显示 tb_name 指定的表的创建语句。支持普通表、超级表和子表。
Shows the SQL statement used to create the specified table. This statement can be used on supertables, standard tables, and subtables.
## SHOW DATABASES
...
...
@@ -85,7 +85,7 @@ SHOW CREATE TABLE [db_name.]tb_name
SHOWDATABASES;
```
显示用户定义的所有数据库。
Shows all user-created databases.
## SHOW DNODES
...
...
@@ -93,7 +93,7 @@ SHOW DATABASES;
SHOWDNODES;
```
显示当前系统中 DNODE 的信息。
Shows all dnodes in the system.
## SHOW FUNCTIONS
...
...
@@ -101,7 +101,7 @@ SHOW DNODES;
SHOWFUNCTIONS;
```
显示用户定义的自定义函数。
Shows all user-defined functions in the system.
## SHOW LICENSE
...
...
@@ -110,9 +110,9 @@ SHOW LICENSE;
SHOWGRANTS;
```
显示企业版许可授权的信息。
Shows information about the TDengine Enterprise Edition license.
注：企业版独有
Note: TDengine Enterprise Edition only.
## SHOW INDEXES
...
...
@@ -120,7 +120,7 @@ SHOW GRANTS;
SHOWINDEXESFROMtbl_name[FROMdb_name];
```
显示已创建的索引。
Shows indices that have been created.
## SHOW LOCAL VARIABLES
...
...
@@ -128,7 +128,7 @@ SHOW INDEXES FROM tbl_name [FROM db_name];
SHOWLOCALVARIABLES;
```
显示当前客户端配置参数的运行值。
Shows the working configuration of the client.
## SHOW MNODES
...
...
@@ -136,7 +136,7 @@ SHOW LOCAL VARIABLES;
SHOWMNODES;
```
显示当前系统中 MNODE 的信息。
Shows information about mnodes in the system.
## SHOW MODULES
...
...
@@ -144,7 +144,7 @@ SHOW MNODES;
SHOWMODULES;
```
显示当前系统中所安装的组件的信息。
Shows information about modules installed in the system.
## SHOW QNODES
...
...
@@ -152,7 +152,7 @@ SHOW MODULES;
SHOWQNODES;
```
显示当前系统中 QNODE （查询节点）的信息。
Shows information about qnodes in the system.
## SHOW SCORES
...
...
@@ -160,9 +160,9 @@ SHOW QNODES;
SHOWSCORES;
```
显示系统被许可授权的容量的信息。
Shows information about the storage space allowed by the license.
注：企业版独有
Note: TDengine Enterprise Edition only.
## SHOW SNODES
...
...
@@ -170,7 +170,7 @@ SHOW SCORES;
SHOWSNODES;
```
显示当前系统中 SNODE （流计算节点）的信息。
Shows information about stream processing nodes (snodes) in the system.
## SHOW STABLES
...
...
@@ -178,7 +178,7 @@ SHOW SNODES;
SHOW[db_name.]STABLES[LIKE'pattern'];
```
显示当前数据库下的所有超级表的信息。可以使用 LIKE 对表名进行模糊匹配。
Shows all supertables in the current database. You can use LIKE for fuzzy matching.
## SHOW STREAMS
...
...
@@ -186,7 +186,7 @@ SHOW [db_name.]STABLES [LIKE 'pattern'];
SHOWSTREAMS;
```
显示当前系统内所有流计算的信息。
Shows information about streams in the system.
## SHOW SUBSCRIPTIONS
...
...
@@ -194,7 +194,7 @@ SHOW STREAMS;
SHOWSUBSCRIPTIONS;
```
显示当前数据库下的所有的订阅关系
Shows all subscriptions in the current database.
## SHOW TABLES
...
...
@@ -202,7 +202,7 @@ SHOW SUBSCRIPTIONS;
SHOW[db_name.]TABLES[LIKE'pattern'];
```
显示当前数据库下的所有普通表和子表的信息。可以使用 LIKE 对表名进行模糊匹配。
Shows all standard tables and subtables in the current database. You can use LIKE for fuzzy matching.
## SHOW TABLE DISTRIBUTED
...
...
@@ -210,7 +210,7 @@ SHOW [db_name.]TABLES [LIKE 'pattern'];
SHOWTABLEDISTRIBUTEDtable_name;
```
显示表的数据分布信息。
Shows how table data is distributed.
## SHOW TAGS
...
...
@@ -218,7 +218,7 @@ SHOW TABLE DISTRIBUTED table_name;
SHOWTAGSFROMchild_table_name[FROMdb_name];
```
显示子表的标签信息。
Shows all tag information in a subtable.
## SHOW TOPICS
...
...
@@ -226,7 +226,7 @@ SHOW TAGS FROM child_table_name [FROM db_name];
SHOWTOPICS;
```
显示当前数据库下的所有主题的信息。
Shows all topics in the current database.
## SHOW TRANSACTIONS
...
...
@@ -234,7 +234,7 @@ SHOW TOPICS;
SHOWTRANSACTIONS;
```
显示当前系统中正在执行的事务的信息
Shows all running transactions in the system.
## SHOW USERS
...
...
@@ -242,7 +242,7 @@ SHOW TRANSACTIONS;
SHOWUSERS;
```
显示当前系统中所有用户的信息。包括用户自定义的用户和系统默认用户。
Shows information about users on the system. This includes user-created users and system-defined users.
## SHOW VARIABLES
...
...
@@ -251,7 +251,7 @@ SHOW VARIABLES;
SHOWDNODEdnode_idVARIABLES;
```
显示当前系统中各节点需要相同的配置参数的运行值，也可以指定 DNODE 来查看其的配置参数。
Shows the working configuration of the parameters that must be the same on each node. You can also specify a dnode to show the working configuration for that node.
## SHOW VGROUPS
...
...
@@ -259,7 +259,7 @@ SHOW DNODE dnode_id VARIABLES;
SHOW[db_name.]VGROUPS;
```
显示当前系统中所有 VGROUP 或某个 db 的 VGROUPS 的信息。
Shows information about all vgroups in the system or about the vgroups for a specified database.
## SHOW VNODES
...
...
@@ -267,4 +267,4 @@ SHOW [db_name.]VGROUPS;
SHOWVNODES[dnode_name];
```
显示当前系统中所有 VNODE 或某个 DNODE 的 VNODE 的信息。
Shows information about all vnodes in the system or about the vnodes for a specified dnode.
The maximum length of password is 128 bytes. The password can include leters, digits, and special characters excluding single quotation marks, double quotation marks, backticks, backslashes, and spaces. The password cannot be empty.
## 删除用户
## Delete a User
```sql
DROPUSERuser_name;
```
## 修改用户信息
## Modify User Information
```sql
ALTERUSERuser_namealter_user_clause
...
...
@@ -35,12 +35,12 @@ alter_user_clause: {
}
```
- PASS：修改用户密码。
- ENABLE：修改用户是否启用。1表示启用此用户，0表示禁用此用户。
- SYSINFO：修改用户是否可查看系统信息。1表示可以查看系统信息，0表示不可以查看系统信息。
- PASS: Modify the user password.
- ENABLE: Specify whether the user is enabled or disabled. 1 indicates enabled and 0 indicates disabled.
- SYSINFO: Specify whether the user can query system information. 1 indicates that the user can query system information and 0 indicates that the user cannot query system information.
## 授权
## Grant Permissions
```sql
GRANTprivilegesONpriv_levelTOuser_name
...
...
@@ -61,15 +61,15 @@ priv_level : {
}
```
对用户授权。
Grant permissions to a user.
授权级别支持到DATABASE，权限有READ和WRITE两种。
Permissions are granted on the database level. You can grant read or write permissions.
TDengine has superusers and standard users. The default superuser name is root. This account has all permissions. You can use the superuser account to create standard users. With no permissions, standard users can create databases and have permissions on the databases that they create. These include deleting, modifying, querying, and writing to their own databases. Superusers can grant users permission to read and write other databases. However, standard users cannot delete or modify databases created by other users.
For non-database objects such as users, dnodes, and user-defined functions, standard users have read permissions only, generally by means of the SHOW statement. Standard users cannot create or modify these objects.
You can create user-defined functions and import them into TDengine.
## Create UDF
## 创建函数
SQL command can be executed on the host where the generated UDF DLL resides to load the UDF DLL into TDengine. This operation cannot be done through REST interface or web console. Once created, any client of the current TDengine can use these UDF functions in their SQL commands. UDF are stored in the management node of TDengine. The UDFs loaded in TDengine would be still available after TDengine is restarted.
When creating UDF, the type of UDF, i.e. a scalar function or aggregate function must be specified. If the specified type is wrong, the SQL statements using the function would fail with errors. The input data type and output data type must be consistent with the UDF definition.
- function_name: The scalar function name to be used in SQL statement which must be consistent with the UDF name and is also the name of the compiled DLL (.so file).
- library_path: The absolute path of the DLL file including the name of the shared object file (.so). The path must be quoted with single or double quotes.
- output_type: The data type of the results of the UDF.
For example, the following SQL statement can be used to create a UDF from `libbitand.so`.
- function_name: The aggregate function name to be used in SQL statement which must be consistent with the udfNormalFunc name and is also the name of the compiled DLL (.so file).
- library_path: The absolute path of the DLL file including the name of the shared object file (.so). The path must be quoted with single or double quotes.
- output_type: The output data type, the value is the literal string of the supported TDengine data type.
- buffer_size: The size of the intermediate buffer in bytes. This parameter is optional.
For example, the following SQL statement can be used to create a UDF from `libl2norm.so`.
对指定列按 INTERVAL 子句定义的时间窗口创建进行预聚合计算，预聚合计算类型由 functions_string 指定。SMA 索引能提升指定时间段的聚合查询的性能。目前，限制一个超级表只能创建一个 SMA INDEX。
Performs pre-aggregation on the specified column over the time window defined by the INTERVAL clause. The type is specified in functions_string. SMA indexing improves aggregate query performance for the specified time period. One supertable can only contain one SMA index.
- WATERMARK: Enter a value between 0ms and 900000ms. The most precise unit supported is milliseconds. The default value is 5 seconds. This option can be used only on supertables.
- MAX_DELAY: Enter a value between 1ms and 900000ms. The most precise unit supported is milliseconds. The default value is the value of interval provided that it does not exceed 900000ms. This option can be used only on supertables. Note: Retain the default value if possible. Configuring a small MAX_DELAY may cause results to be frequently pushed, affecting storage and query performance.
Creates a text index for the specified column. FULLTEXT indexing improves performance for queries with text filtering. The index_option syntax is not supported for FULLTEXT indexing. FULLTEXT indexing is supported for JSON tag columns only. Multiple columns cannot be indexed together. However, separate indices can be created for each column.
## 删除索引
## Delete an Index
```sql
DROPINDEXindex_name;
```
## 查看索引
## View Indices
````sql
```sql
SHOW INDEXES FROM tbl_name [FROM db_name];
````
显示在所指定的数据库或表上已创建的索引。
Shows indices that have been created for the specified database or table.
In a complex environment, connections and query tasks may encounter errors or fail to return in a reasonable time. If this occurs, you can terminate the connection or task.
## 终止连接
## Terminate a Connection
```sql
KILLCONNECTIONconn_id;
```
conn_id 可以通过 `SHOW CONNECTIONS` 获取。
You can use the SHOW CONNECTIONS statement to find the conn_id.
## 终止查询
## Terminate a Query
```sql
SHOWQUERYquery_id;
```
query_id 可以通过 `SHOW QUERIES` 获取。
You can use the SHOW QUERIES statement to find the query_id.
## 终止事务
## Terminate a Transaction
```sql
KILLTRANSACTIONtrans_id
```
trans_id 可以通过 `SHOW TRANSACTIONS` 获取。
You can use the SHOW TRANSACTIONS statement to find the trans_id.
If metadata becomes desynchronized among multiple clients, you can use this command to clear the client-side cache. Clients then obtain the latest metadata from the server.
| 3 | _ROWTS pseudocolumn | Added | Indicates the primary key. Alias of _C0.
| 4 | INFORMATION_SCHEMA | Added | Database for system metadata containing all schema definitions
| 5 | PERFORMANCE_SCHEMA | Added | Database for system performance information.
| 6 | Connection queries | Deprecated | Connection queries are no longer supported. The syntax and interfaces are deprecated.
| 7 | Mixed operations | Enhanced | Mixing scalar and vector operations in queries has been enhanced and is supported in all SELECT clauses.
| 8 | Tag operations | Added | Tag columns can be used in queries and clauses like data columns.
| 9 | Timeline clauses and time functions in supertables | Enhanced | When PARTITION BY is not used, data in supertables is merged into a single timeline.
## SQL Syntax
The following data types can be used in the schema for standard tables.
| 1 | ALTER ACCOUNT | Deprecated| This Enterprise Edition-only statement has been removed. It returns the error "This statement is no longer supported."
| 2 | ALTER ALL DNODES | Added | Modifies the configuration of all dnodes.
| 3 | ALTER DATABASE | Modified | Deprecated<ul><li>QUORUM: Specified the required number of confirmations. STRICT is now used to specify strong or weak consistency. The STRICT parameter cannot be modified. </li><li>BLOCKS: Specified the memory blocks used by each vnode. BUFFER is now used to specify the size of the write cache pool for each vnode. </li><li>UPDATE: Specified whether update operations were supported. All databases now support updating data in certain columns. </li><li>CACHELAST: Specified how to cache the newest row of data. CACHEMODEL now replaces CACHELAST. </li><li>COMP: Cannot be modified. <br/>Added</li><li>CACHEMODEL: Specifies whether to cache the latest subtable data. </li><li>CACHESIZE: Specifies the size of the cache for the newest subtable data. </li><li>WAL_FSYNC_PERIOD: Replaces the FSYNC parameter. </li><li>WAL_LEVEL: Replaces the WAL parameter. <br/>Modified</li><li>REPLICA: Cannot be modified. </li><li>KEEP: Now supports units. </li></ul>
| 4 | ALTER STABLE | Modified | Deprecated<ul><li>CHANGE TAG: Modified the name of a tag. Replaced by RENAME TAG. <br/>Added</li><li>RENAME TAG: Replaces CHANGE TAG. </li><li>COMMENT: Specifies comments for a supertable. </li></ul>
| 5 | ALTER TABLE | Modified | Deprecated<ul><li>CHANGE TAG: Modified the name of a tag. Replaced by RENAME TAG. <br/>Added</li><li>RENAME TAG: Replaces CHANGE TAG. </li><li>COMMENT: Specifies comments for a standard table. </li><li>TTL: Specifies the time-to-live for a standard table. </li></ul>
| 6 | ALTER USER | Modified | Deprecated<ul><li>PRIVILEGE: Specified user permissions. Replaced by GRANT and REVOKE. <br/>Added</li><li>ENABLE: Enables or disables a user. </li><li>SYSINFO: Specifies whether a user can query system information. </li></ul>
| 7 | COMPACT VNODES | Not supported | Compacted the data on a vnode. Not supported.
| 8 | CREATE ACCOUNT | Deprecated| This Enterprise Edition-only statement has been removed. It returns the error "This statement is no longer supported."
| 9 | CREATE DATABASE | Modified | Deprecated<ul><li>BLOCKS: Specified the number of blocks for each vnode. BUFFER is now used to specify the size of the write cache pool for each vnode. </li><li>CACHE: Specified the size of the memory blocks used by each vnode. BUFFER is now used to specify the size of the write cache pool for each vnode. </li><li>CACHELAST: Specified how to cache the newest row of data. CACHEMODEL now replaces CACHELAST. </li><li>DAYS: The length of time to store in a single file. Replaced by DURATION. </li><li>FSYNC: Specified the fsync interval when WAL was set to 2. Replaced by WAL_FSYNC_PERIOD. </li><li>QUORUM: Specified the number of confirmations required. STRICT is now used to specify strong or weak consistency. </li><li>UPDATE: Specified whether update operations were supported. All databases now support updating data in certain columns. </li><li>WAL: Specified the WAL level. Replaced by WAL_LEVEL. <br/>Added</li><li>BUFFER: Specifies the size of the write cache pool for each vnode. </li><li>CACHEMODEL: Specifies whether to cache the latest subtable data. </li><li>CACHESIZE: Specifies the size of the cache for the newest subtable data. </li><li>DURATION: Replaces DAYS. Now supports units. </li><li>PAGES: Specifies the number of pages in the metadata storage engine cache on each vnode. </li><li>PAGESIZE: specifies the size (in KB) of each page in the metadata storage engine cache on each vnode. </li><li>RETENTIONS: Specifies the aggregation interval and retention period </li><li>STRICT: Specifies whether strong data consistency is enabled. </li><li>SINGLE_STABLE: Specifies whether a database can contain multiple supertables. </li><li>VGROUPS: Specifies the initial number of vgroups when a database is created. </li><li>WAL_FSYNC_PERIOD: Replaces the FSYNC parameter. </li><li>WAL_LEVEL: Replaces the WAL parameter. </li><li>WAL_RETENTION_PERIOD: specifies the time after which WAL files are deleted. This parameter is used for data subscription. </li><li>WAL_RETENTION_SIZE: specifies the size at which WAL files are deleted. This parameter is used for data subscription. </li><li>WAL_ROLL_PERIOD: Specifies the WAL rotation period. </li><li>WAL_SEGMENT_SIZE: specifies the maximum size of a WAL file. <br/>Modified</li><li>KEEP: Now supports units. </li></ul>
| 10 | CREATE DNODE | Modified | Now supports specifying hostname and port separately<ul><li>CREATE DNODE dnode_host_name PORT port_val</li></ul>
| 11 | CREATE INDEX | Added | Creates an SMA index.
| 12 | CREATE MNODE | Added | Creates an mnode.
| 13 | CREATE QNODE | Added | Creates a qnode.
| 14 | CREATE STABLE | Modified | New parameter added<li>COMMENT: Specifies comments for the supertable. </li>
| 15 | CREATE STREAM | Added | Creates a stream.
| 16 | CREATE TABLE | Modified | New parameters added<ul><li>COMMENT: Specifies comments for the table </li><li>WATERMARK: Specifies the window closing time. </li><li>MAX_DELAY: Specifies the maximum delay for pushing stream processing results. </li><li>ROLLUP: Specifies aggregate functions to roll up. Rolling up a function provides downsampled results based on multiple axes. </li><li>SMA: Provides user-defined precomputation of aggregates based on data blocks. </li><li>TTL: Specifies the time-to-live for a standard table. </li></ul>
| 17 | CREATE TOPIC | Added | Creates a topic.
| 18 | DROP ACCOUNT | Deprecated| This Enterprise Edition-only statement has been removed. It returns the error "This statement is no longer supported."
| 19 | DROP CONSUMER GROUP | Added | Deletes a consumer group.
| 29 | KILL STREAM | Deprecated | Terminated a continuous query. The continuous query feature has been replaced with the stream processing feature.
| 30 | MERGE VGROUP | Added | Merges vgroups.
| 31 | REVOKE | Added | Revokes permissions from a user.
| 32 | SELECT | Modified | <ul><li>SELECT does not use the implicit results column. Output columns must be specified in the SELECT clause. </li><li>DISTINCT support is enhanced. In previous versions, DISTINCT only worked on the tag column and could not be used with JOIN or GROUP BY. </li><li>JOIN support is enhanced. The following are now supported after JOIN: a WHERE clause with OR, operations on multiple tables, and GROUP BY on multiple tables. </li><li>Subqueries after FROM are enhanced. Levels of nesting are no longer restricted. Subqueries can be used with UNION ALL. Other syntax restrictions are eliminated. </li><li>All scalar functions can be used after WHERE. </li><li>GROUP BY is enhanced. You can group by any scalar expression or combination thereof. </li><li>SESSION can be used on supertables. When PARTITION BY is not used, data in supertables is merged into a single timeline. </li><li>STATE_WINDOW can be used on supertables. When PARTITION BY is not used, data in supertables is merged into a single timeline. </li><li>ORDER BY is enhanced. It is no longer required to use ORDER BY and GROUP BY together. There is no longer a restriction on the number of order expressions. NULLS FIRST and NULLS LAST syntax has been added. Any expression that conforms to the ORDER BY semantics can be used. </li><li>Added PARTITION BY syntax. PARTITION BY replaces GROUP BY tags. </li></ul>
| 33 | SHOW ACCOUNTS | Deprecated | This Enterprise Edition-only statement has been removed. It returns the error "This statement is no longer supported."
| 34 | SHOW APPS | Added | Shows all clients (such as applications) that connect to the cluster.
| 35 | SHOW CONSUMERS | Added | Shows information about all active consumers in the system.
| 36 | SHOW DATABASES | Modified | Only shows database names.
| 37 | SHOW FUNCTIONS | Modified | Only shows UDF names.
| 38 | SHOW LICENCE | Added | Alias of SHOW GRANTS.
| 39 | SHOW INDEXES | Added | Shows indices that have been created.
| 40 | SHOW LOCAL VARIABLES | Added | Shows the working configuration of the client.
| 41 | SHOW MODULES | Deprecated | Shows information about modules installed in the system.
| 42 | SHOW QNODES | Added | Shows information about qnodes in the system.
| 43 | SHOW STABLES | Modified | Only shows supertable names.
| 44 | SHOW STREAMS | Modified | This statement previously showed continuous queries. The continuous query feature has been replaced with the stream processing feature. This statement now shows streams that have been created.
| 45 | SHOW SUBSCRIPTIONS | Added | Shows all subscriptions in the current database.
| 46 | SHOW TABLES | Modified | Only shows table names.
| 47 | SHOW TABLE DISTRIBUTED | Added | Shows how table data is distributed. This replaces the `SELECT _block_dist() FROM { tb_name | stb_name }` command.
| 48 | SHOW TOPICS | Added | Shows all subscribed topics in the current database.
| 49 | SHOW TRANSACTIONS | Added | Shows all running transactions in the system.
| 50 | SHOW DNODE VARIABLES | Added | Shows the configuration of the specified dnode.
| 51 | SHOW VNODES | Not supported | Shows information about vnodes in the system. Not supported.
| 52 | SPLIT VGROUP | Added | Splits a vgroup into two vgroups.
| 53 | TRIM DATABASE | Added | Deletes data that has expired and orders the remaining data in accordance with the storage configuration.
This section explains the syntax of SQL to perform operations on databases, tables and STables, insert data, select data and use functions. We also provide some tips that can be used in TDengine SQL. If you have previous experience with SQL this section will be fairly easy to understand. If you do not have previous experience with SQL, you'll come to appreciate the simplicity and power of SQL.
This section explains the syntax of SQL to perform operations on databases, tables and STables, insert data, select data and use functions. We also provide some tips that can be used in TDengine SQL. If you have previous experience with SQL this section will be fairly easy to understand. If you do not have previous experience with SQL, you'll come to appreciate the simplicity and power of SQL. TDengine SQL has been enhanced in version 3.0, and the query engine has been rearchitected. For information about how TDengine SQL has changed, see [Changes in TDengine 3.0](/taos-sql/changes).
TDengine SQL is the major interface for users to write data into or query from TDengine. For ease of use, the syntax is similar to that of standard SQL. However, please note that TDengine SQL is not standard SQL. For instance, TDengine doesn't provide a delete function for time series data and so corresponding statements are not provided in TDengine SQL.
TDengine SQL is the major interface for users to write data into or query from TDengine. It uses standard SQL syntax and includes extensions and optimizations for time-series data and services. The maximum length of a TDengine SQL statement is 1 MB. Note that keyword abbreviations are not supported. For example, DELETE cannot be entered as DEL.
Syntax Specifications used in this chapter:
- The content inside <\> needs to be input by the user, excluding <\> itself.
- Keywords are given in uppercase, although SQL is not case-sensitive.
- Information that you input is given in lowercase.
-\[\] means optional input, excluding [] itself.
- | means one of a few options, excluding | itself.
- … means the item prior to it can be repeated multiple times.
To better demonstrate the syntax, usage and rules of TAOS SQL, hereinafter it's assumed that there is a data set of data from electric meters. Each meter collects 3 data measurements: current, voltage, phase. The data model is shown below:
The data set includes the data collected by 4 meters, the corresponding table name is d1001, d1002, d1003 and d1004 based on the data model of TDengine.
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