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 or querying data, timestamp must follow below rules:
-时间可以加减，比如 now-2h，表明查询时刻向前推 2 个小时（最近 2 小时）。数字后面的时间单位可以是 b(纳秒)、u(微秒)、a(毫秒)、s(秒)、m(分)、h(小时)、d(天)、w(周)。 比如 `select * from t1 where ts > now-2w and ts <= now-1w`，表示查询两周前整整一周的数据。在指定降采样操作（down sampling）的时间窗口（interval）时，时间单位还可以使用 n (自然月) 和 y (自然年)。
-the format must be `YYYY-MM-DD HH:mm:ss.MS`, the default time precision is millisecond (ms), for example `2017-08-12 18:25:58.128`
-internal function `now` can be used to get the current timestamp of the client side
-the current timestamp of the client side is applied when `now` is used to insert data
- Epoch Time：timestamp can also be a long integer number, which means the number of seconds, milliseconds or nanoseconds, depending on the time precision, from 1970-01-01 00:00:00.000 (UTC/GMT)
-timestamp can be applied with add/substract operation, for example `now-2h` means 2 hours back from the time at which query is executed，the unit can be b(nanosecond), u(microsecond), a(millisecond), s(second), m(minute), h(hour), d(day), w(week.。 So `select * from t1 where ts > now-2w and ts <= now-1w` means the data between two weeks ago and one week ago. The time unit can also be n (calendar month) or y (calendar year) when specifying the time window for down sampling operation.
Time precision in TDengine can be set by the `PRECISION` parameter when executing `CREATE DATABASE`, like below, the default time precision is millisecond.
| 1 | TIMESTAMP | 8 | Default precision is millisecond, microsecond and nanosecond are also supported |
| 2 | INT | 4 | Integer, the value range is [-2^31+1, 2^31-1], while -2^31 is treated as NULL |
| 3 | BIGINT | 8 | Long integer, the value range is [-2^63+1, 2^63-1], while -2^63 is treated as NULL |
| 4 | FLOAT | 4 | Floating point number, the effective number of digits is 6-7, the value range is [-3.4E38, 3.4E38] |
| 5 | DOUBLE | 8 | double precision floating point number, the effective number of digits is 15-16, the value range is [-1.7E308, 1.7E308] |
| 6 | 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 `\'` |
| 7 | SMALLINT | 2 | Short integer, the value range is [-32767, 32767], while -32768 is treated as NULL |
| 8 | TINYINT | 1 | Single-byte integer, the value range is [-127, 127], while -128 is treated as NLLL |
| 9 | BOOL | 1 | Bool, the value range is {true, false} |
| 10 | NCHAR | User Defined| Multiple-Byte string that can include 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. Error will be reported the string value exceeds the length defined. |
| 11 | JSON | | json type can only be used on tag, a tag of json type is excluded with any other tags of any other type |
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.
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.
1. KEEP specifies the number of days for which the data in the database to be created will be kept, 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 data with an existing timestamp will be dropped silently.
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 part of columns for a row is allowed, the columns for which no value is specified will be kept as no change
3.The maximum length of database name is 33 bytes.
4.The maximum length of a SQL statement is 65,480 bytes.
5.For more parameters that can be used when creating a database, like cache, blocks, days, keep, minRows, maxRows, wal, fsync, update, cacheLast, replica, quorum, maxVgroupsPerDb, ctime, comp, prec, Please refer to [Configuration Parameters](/reference/config/).
:::
## 显示系统当前参数
## Show Current Configuration
```
SHOW VARIABLES;
```
## 使用数据库
## Specify The Database In Use
```
USE db_name;
```
使用/切换数据库（在 REST 连接方式下无效）。
:::note
This way is not applicable when using a REST connection
:::
## 删除数据库
## Drop Database
```
DROP DATABASE [IF EXISTS] db_name;
```
删除数据库。指定 Database 所包含的全部数据表将被删除，谨慎使用！
:::note
All data in the database will be deleted too. This command must be used with caution.
:::
## Change Database Configuration
## 修改数据库参数
Some examples are shown below to demonstrate how to change the configuration of a database. Please be noted that some configuration parameters can be changed after the database is created, but some others can't, for details of the configuration parameters of database please refer to [Configuration Parameters](/reference/config/).
说明：缓存最近行，将显著改善 LAST_ROW 函数的性能表现；缓存每列的最近非 NULL 值，将显著改善无特殊影响（WHERE、ORDER BY、GROUP BY、INTERVAL）下的 LAST 函数的性能表现。
CACHELAST parameter specifies whether and how the latest data of a sub table is cached.
:::tip
以上所有参数修改后都可以用 show databases 来确认是否修改成功。另外，从 2.1.3.0 版本开始，修改这些参数后无需重启服务器即可生效。
:::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/).
This command is useful when migrating the data from one TDengine cluster to another one. Firstly this command can be used to get the CREATE statement, which in turn can be used in another TDengine to create an exactly same database.
1. The first column of a table must be in TIMESTAMP type, and it will be set as primary key automatically
2. The maximum length of table name is 192 bytes.
3. The maximum length of each row is 16k bytes, please be notes that the extra 2 bytes used by each BINARY/NCHAR column are also counted in.
4. The name of sub-table can only be consisted of English characters, digits and underscore, and can't be started with digit. Table names are case insensitive.
5. The maximum length in bytes must be specified when using BINARY or NCHAR type.
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 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.
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.
:::
### 以超级表为模板创建数据表
### Create Table Using STable As Template
```
CREATE TABLE [IF NOT EXISTS] tb_name USING stb_name TAGS (tag_value1, ...);
```
以指定的超级表为模板，指定 TAGS 的值来创建数据表。
The above command creates a sub table using the specified super table as template and the specified tab values.
### 以超级表为模板创建数据表，并指定具体的 TAGS 列
### Create Table Using STable As Template With A Part of Tags
```
CREATE TABLE [IF NOT EXISTS] tb_name USING stb_name (tag_name1, ...) TAGS (tag_value1, ...);
This way is useful when migrating the data in one TDengine cluster to another one because it can be used to create exactly same tables in the target database.
## 获取表的结构信息
## Show Table Definition
```
DESCRIBE tb_name;
```
## 修改表定义
## Change Table Definition
### 表增加列
### Add A Column
```
ALTER TABLE tb_name ADD COLUMN field_name data_type;
If a table is created using a super table as template, the table definition can only be changed on the corresponding super table, but the change will be automatically applied to all the sub tables created using this super table as template. For tables created in normal way, the table definition can be changed directly on the table.
:::
### 表修改列宽
### Change Column Length
```
ALTER TABLE tb_name MODIFY COLUMN field_name data_type(length);
The the type of a column is variable length, like BINARY or NCHAR, this way can be used to change (or increase) the length of the column.
### 修改子表标签值
:::note
If a table is created using a super table as template, the table definition can only be changed on the corresponding super table, but the change will be automatically applied to all the sub tables created using this super table as template. For tables created in normal way, the table definition can be changed directly on the table.
:::
### Change Tag Value Of Sub Table
```
ALTER TABLE tb_name SET TAG tag_name=new_tag_value;
```
如果表是通过超级表创建，可以使用此指令修改其标签值
This command can be used to change the tag value if the table is created using a super table as template.
The SQL statement of creating STable is similar to that of creating table, but a special column named as `TAGS` must be specified with the names and types of the tags.
1. The tag types specified in TAGS should NOT be timestamp. Since 2.1.3.0 timestamp type can be used in TAGS column, but its value must be fixed and arithmetic operation can't be applied on it.
2. The tag names specified in TAGS should NOT be same as other columns.
3. The tag names specified in TAGS should NOT be same as any reserved keywords.(Please refer to [keywords](/taos-sql/keywords/)
4. The maximum number of tags specified in TAGS is 128, but there must be at least one tag, and the total length of all tag columns should NOT exceed 16KB.
:::
## 删除超级表
## Drop STable
```
DROP STABLE [IF EXISTS] stb_name;
```
删除 STable 会自动删除通过 STable 创建的子表。
All the sub-tables created using the deleted stable will be deleted automatically.
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, number of tables created using this STable.
This command is useful in migrating data from one TDengine cluster to another one because it can be used to create an exactly same STable in the target database.
## 获取超级表的结构信息
## Get STable Definition
```
DESCRIBE stb_name;
```
## 修改超级表普通列
## Change Columns Of STable
### 超级表增加列
### Add A Column
```
ALTER STABLE stb_name ADD COLUMN field_name data_type;
```
### 超级表删除列
### Remove A Column
```
ALTER STABLE stb_name DROP COLUMN field_name;
```
### 超级表修改列宽
### Change Column Length
```
ALTER STABLE stb_name MODIFY COLUMN field_name data_type(length);
This command is used to add a new tag for a STable and specify the tag type.
### 删除标签
### Remove A Tag
```
ALTER STABLE stb_name DROP TAG tag_name;
```
删除超级表的一个标签，从超级表删除某个标签后，该超级表下的所有子表也会自动删除该标签。
The tag will be removed automatically from all the sub tables crated using the super table as template once a tag is removed from a super table.
### 修改标签名
### Change A Tag
```
ALTER STABLE stb_name CHANGE TAG old_tag_name new_tag_name;
```
修改超级表的标签名，从超级表修改某个标签名后，该超级表下的所有子表也会自动更新该标签名。
The tag name will be changed automatically from all the sub tables crated using the super table as template once a tag name is changed for a super table.
### 修改标签列宽度
### Change Tag Length
```
ALTER STABLE stb_name MODIFY TAG tag_name data_type(length);
Changing tag value can be applied to only sub tables. 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 sub tables.
2. 在使用“插入多条记录”方式写入数据时，不能把第一列的时间戳取值都设为 NOW，否则会导致语句中的多条记录使用相同的时间戳，于是就可能出现相互覆盖以致这些数据行无法全部被正确保存。其原因在于，NOW 函数在执行中会被解析为所在 SQL 语句的实际执行时间，出现在同一语句中的多个 NOW 标记也就会被替换为完全相同的时间戳取值。
3. 允许插入的最老记录的时间戳，是相对于当前服务器时间，减去配置的 keep 值（数据保留的天数）；允许插入的最新记录的时间戳，是相对于当前服务器时间，加上配置的 days 值（数据文件存储数据的时间跨度，单位为天）。keep 和 days 都是可以在创建数据库时指定的，缺省值分别是 3650 天和 10 天。
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 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.
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 part 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.
:::
## 向多个表插入记录
## Insert Into Multiple Tables
可以在一条语句中，分别向多个表插入一条或多条记录，并且也可以在插入过程中指定列。例如：
One or multiple rows can be inserted into multiple tables in single SQL statement, with or without specifying specific columns.
If it's not sure whether the table already exists, the table can be created automatically while inserting using below SQL statement. To use this functionality, a STable must be used as template and tag values must be provided.
Prior to version 2.0.20.5, when using `INSERT` to create table automatically and specify 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 same SQL statement, however, these two ways of specifying column names can't be mixed.
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 below data:
```
'2021-07-13 14:07:34.630', '10.2', '219', '0.32'
'2021-07-13 14:07:35.779', '10.15', '217', '0.33'
```
那么通过如下指令可以把这个文件中的数据写入子表中：
Then data in this file can be inserted by below SQL statement:
```
```sql
INSERTINTOd1001FILE'/tmp/csvfile.csv';
```
## 插入来自文件的数据记录，并自动建表
## CreateTables Automatically and Insert Rows From File
For SQL statement like `insert`, 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)
taos> SHOW TABLES;
...
...
@@ -147,3 +161,5 @@ taos> SHOW TABLES;
d1001 | 2020-08-06 17:52:02.097 | 4 | meters |
Query OK, 1 row(s) in set (0.001091s)
```
From the above experiment, we can see that even though the value to be inserted is invalid but the table is still created.
Aggregate by time window is supported in TDengine. For example, each temperature sensor reports the temperature every second, the average temperature every 10 minutes can be retrieved by query with time window.
Window related clauses are used to divide the data set to be queried into subsets and then aggregate. 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 window.
`INTERVAL` claused is used to generate time windows of same time interval, `SLIDING` 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 continuous query both the size of 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 range 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 window.
`INTERVAL` and `SLIDING` should be used with aggregate functions and selection functions. Below SQL statement is illegal because no aggregate or selection function is used with `INTERVAL`.
```
SELECT * FROM temp_tb_1 INTERVAL(1m);
```
SLIDING 的向前滑动的时间不能超过一个窗口的时间范围。以下语句非法：
The time step specified by `SLIDING` can't exceed the time interval specified by `INTERVAL`. Below SQL statement 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 same as that specified by `INTERVAL`, sliding window is actually flip window. The minimum time range specified by `INTERVAL` is 10 milliseconds (10a) prior to version 2.1.5.0. From 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 be noted that the `timezone` parameter should be configured to same value in the `taos.cfg` configuration file on client side and server side.
In case of using integer, bool, or string to represent the device status at a moment, the continuous rows with same status belong to same 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.
使用 STATE_WINDOW 来确定状态窗口划分的列。例如：
`STATE_WINDOW` is used to specify the column based on which to define status window, for example：
```
SELECT COUNT(*), FIRST(ts), status FROM temp_tb_1 STATE_WINDOW(status);
The primary key, i.e. timestamp, is used to determine which session window the row belongs to. If the time interval between two adjacent rows is within the time range specified by `tol_val`, they belong to same session window; otherwise they belong to two different time windows. As shown in the figure below, if the limit of time interval for 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 withint 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);
```
## More On Window Aggregate
这种类型的查询语法如下：
### Syntax
```
The full syntax of aggregate by window is as following:
```sql
SELECTfunction_listFROMtb_name
[WHEREwhere_condition]
[SESSION(ts_col,tol_val)]
...
...
@@ -71,39 +71,38 @@ SELECT function_list FROM stb_name
2. VALUE 填充：固定值填充，此时需要指定填充的数值。例如：FILL(VALUE, 1.23)。
3. PREV 填充：使用前一个非 NULL 值填充数据。例如：FILL(PREV)。
4. NULL 填充：使用 NULL 填充数据。例如：FILL(NULL)。
5. LINEAR 填充：根据前后距离最近的非 NULL 值做线性插值填充。例如：FILL(LINEAR)。
6. NEXT 填充：使用下一个非 NULL 值填充数据。例如：FILL(NEXT)。
- Aggregate functions and selection 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 ouput can't be used, for example DIFF or arithmetic operations.
-`LAST_ROW` can't be used together with window aggregate.
- Scalar functions, like CEIL/FLOOR, can't be used with window aggregate.
-`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)`
:::info
1. 使用 FILL 语句的时候可能生成大量的填充输出，务必指定查询的时间区间。针对每次查询，系统可返回不超过 1 千万条具有插值的结果。
2. 在时间维度聚合中，返回的结果中时间序列严格单调递增。
3. 如果查询对象是超级表，则聚合函数会作用于该超级表下满足值过滤条件的所有表的数据。如果查询中没有使用 GROUP BY 语句，则返回的结果按照时间序列严格单调递增；如果查询中使用了 GROUP BY 语句分组，则返回结果中每个 GROUP 内不按照时间序列严格单调递增。
1. Huge volume of interpolation output may be returned using `FILL`, so it's recommended to specify the time range when using `FILL`. The maximum interpolation values that can be returned in single query is 10,000,000.
2. The result set is in the ascending order of timestamp in aggregate by time window aggregate.
3. If aggregate by window is used on stable, the aggregate function is performed on all the rows matching the filter conditions. If `GROUP BY` is not used in the query, the result set will be returned in ascending order of timestamp; otherwise the result set is not exactly in the order of ascending timestamp in each group.
:::
:::
Aggregate by time window is also used in continuous query, please refer to [Continuous Query](/develop/continuous-query).
The average current, maximum current and median of current in every 10 minutes of the past 24 hours can be calculated using below SQL statement, with missing value filled with the previous non-NULL value.
```
SELECT AVG(current), MAX(current), APERCENTILE(current, 50) FROM meters
1. Only English characters, digits and underscore are allowed
2. Can't be started with digits
3. Case Insensitive without escape character "\`"
4. Identifier with escape character "\`"
To support more flexible table or column names, a new escape character "\`" is introduced. For more details please refer to [escape](/taos-sql/escape).
## GROUP BY 的限制
## Password Rule
TAOS SQL 支持对标签、TBNAME 进行 GROUP BY 操作，也支持普通列进行 GROUP BY，前提是：仅限一列且该列的唯一值小于 10 万个。注意：group by 不支持 float,double 类型。
The legal character set is `[a-zA-Z0-9!?$%^&*()_–+={[}]:;@~#|<,>.?/]`.
## IS NOT NULL 的限制
## General Limits
IS NOT NULL 与不为空的表达式适用范围。
- Maximum length of database name is 32 bytes
- 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 from version 2.1.7.0 , before which the limit is 16K bytes. Please be noted that the upper limit includes the extra 2 bytes consumed by each column of BINARY/NCHAR type.
- Maximum of column name is 64.
- Maximum number of columns is 4096. There must be at least 2 columns, and the first column must be timestamp.
- Maximum length of tag name is 64.
- Maximum number of tags is 128. There must be at least 1 tag. The total length of tag values should not exceed 16K bytes.
- Maximum length of singe SQL statement is 1048576, i.e. 1 MB bytes. It can be configured in the parameter `maxSQLLength` in the client side, the applicable range is [65480, 1048576].
- At most 4096 columns (or 1024 prior to 2.1.7.0) can be returned by `SELECT`, functions in the query statement may constitute columns. Error will be returned if the limit is exceeded.
- Maximum numbers of databases, stables, tables are only depending on the system resources.
- Maximum of database name is 32 bytes, can't include "." and special characters.
- Maximum replica number of database is 3
- Maximum length of user name is 23 bytes
- Maximum length of password is 15 bytes
- Maximum number of rows depends on the storage space only.
- Maximum number of tables depends on the number of nodes only.
- Maximum number of databases depends on the number of nodes only.
- Maximum number of vnodes for single database is 64.
IS NOT NULL 支持所有类型的列。不为空的表达式为 <\>""，仅对非数值类型的列适用。
## Restrictions of `GROUP BY`
## ORDER BY 的限制
`GROUP BY` can be performed on tags and `TBNAME`. It can be performed on data columns too, with one restriction that only one column and the number of unique values on that column is lower than 100,000. Please be noted that `GROUP BY` can't be performed on float or double type.
- 非超级表只能有一个 order by.
- 超级表最多两个 order by， 并且第二个必须为 ts.
- order by tag，必须和 group by tag 一起，并且是同一个 tag。 tbname 和 tag 一样逻辑。 只适用于超级表
- order by 普通列，必须和 group by 一起或者和 top/bottom 一起，并且是同一个普通列。 适用于超级表和普通表。如果同时存在 group by 和 top/bottom 一起，order by 优先必须和 group by 同一列。
- order by ts. 适用于超级表和普通表。
- order by ts 同时含有 group by 时 针对 group 内部用 ts 排序
## Restrictions of `IS NOT NULL`
## 表(列)名合法性说明
`IS NOT NULL` can be used on any data type of columns. The non-empty string evaluation expression, i.e. `<\>""` can only be used on non-numeric data types.
The name of a table or column can only be composed of ASCII characters, digits and underscore, while digit can't be used as the beginning. 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.
### Name Restrictions After Escaping
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 name. 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.
For example:
\`aBc\` and \`abc\` are different table or column names, but "abc" and "aBc" are same names because internally they are all "abc".
:::note
转义字符中的内容必须是可打印字符。
The characters inside escape characters must be printable characters.
:::
### 支持版本
支持转义符的功能从 2.3.0.1 版本开始。
\ No newline at end of file
### Applicable Versions
Escape character "\`" is available from version 2.3.0.1.
2. 对于%和\_，因为在 like 里这两个字符是通配符，所以在模式匹配 like 里用`\%`%和`\_`表示字符里本身的%和\_，如果在 like 模式匹配上下文之外使用`\%`或`\_`，则它们的计算结果为字符串`\%`和`\_`，而不是%和\_。
1.If there are escape characters in identifiers (database name, table name, column name)
- Identifier without ``: Error will be returned because identifier must be constituted of digits, ASCII characters or underscore and can't be started with digits
- Identifier quoted with ``： Original content is kept, no escaping
2.If there are escape characters in values
- The escape characters will be escaped as the above table. If the escape character doesn't match any supported one, the escape character "\" will be ignored.
- "%" and "\_" are used as wildcards in `like`. `\%` and `\_` should be used to represent literal "%" and "\_" in `like`,. If `\%` and `\_` are used out of `like` context, the evaluation result is "`\%`"and "`\_`", instead of "%" and "\_".
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.
This document explains the syntax, select, functions and some tips that can be used in TAOS SQL. It would be easier to understand with some fundamental knowledge of SQL.
TAOS SQL is the major interface for users to write data into or query from TDengine. For users to easily use, syntax similar to standard SQL is provided. However, please be noted that TAOS SQL is not standard SQL. Besides, because TDengine doesn't provide the functionality of deleting time series data, corresponding statements are not provided in TAOS SQL.
TAOS SQL 不支持关键字的缩写，例如 DESCRIBE 不能缩写为 DESC。
TAOS SQL doesn't support abbreviation for keywords, for example `DESCRIBE` can't be abbreviated as `DESC`.
本章节 SQL 语法遵循如下约定：
Syntax Specifications used in this chapter:
-<\> 里的内容是用户需要输入的，但不要输入 <\> 本身
-\[\]表示内容为可选项，但不能输入 [] 本身
- | 表示多选一，选择其中一个即可，但不能输入 | 本身
- … 表示前面的项可重复多个
-The content inside <\> needs to be input by the user, excluding <\> itself.
-\[\]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 meters. Assuming each meter collects 3 data: current, voltage, phase. The data model is as below:
The data set includes the data collected by 4 meters, the corresponding table name is d1001, d1002, d1003, d1004 respectively based on the data model of TDengine.
