07-function.md

---
sidebar_label: Functions
title: Functions
---

## Aggregate Functions

Aggregate query is supported in TDengine by following aggregate functions and selection functions.

### COUNT

```
SELECT COUNT([*|field_name]) FROM tb_name [WHERE clause];
```

**Description**:Get the number of rows or the number of non-null values in a table or a super table.

**Return value type**:Long integer INT64

**Applicable column types**:All

**Applicable table types**: table, super table, sub table

**More explanation**:

- Wildcard (\*) can be used to represent all columns, it's used to get the number of all rows
- The number of non-NULL values will be returned if this function is used on a specific column

**Examples**:

```
taos> SELECT COUNT(*), COUNT(voltage) FROM meters;
    count(*)        |    count(voltage)     |
================================================
                    9 |                     9 |
Query OK, 1 row(s) in set (0.004475s)

taos> SELECT COUNT(*), COUNT(voltage) FROM d1001;
    count(*)        |    count(voltage)     |
================================================
                    3 |                     3 |
Query OK, 1 row(s) in set (0.001075s)
```

### AVG

```
SELECT AVG(field_name) FROM tb_name [WHERE clause];
```

**Description**:Get the average value of a column in a table or stable

**Return value type**:Double precision floating number

**Applicable column types**:Data types except for timestamp, binary, nchar and bool

**Applicable table types**:table, stable

**Examples**:

```
taos> SELECT AVG(current), AVG(voltage), AVG(phase) FROM meters;
    avg(current)        |       avg(voltage)        |        avg(phase)         |
====================================================================================
            11.466666751 |             220.444444444 |               0.293333333 |
Query OK, 1 row(s) in set (0.004135s)

taos> SELECT AVG(current), AVG(voltage), AVG(phase) FROM d1001;
    avg(current)        |       avg(voltage)        |        avg(phase)         |
====================================================================================
            11.733333588 |             219.333333333 |               0.316666673 |
Query OK, 1 row(s) in set (0.000943s)
```

### TWA

```
SELECT TWA(field_name) FROM tb_name WHERE clause;
```

**Description**:Time weighted average on a specific column within a time range

**Return value type**:Double precision floating number

**Applicable column types**:Data types except for timestamp, binary, nchar and bool

**Applicable table types**:table, stable

**More explanations**:

- From version 2.1.3.0, function TWA can be used on stble with `GROUP BY`, i.e. timelines generated by `GROUP BY tbname` on a stable.

### IRATE

```
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 precision floating number

**Applicable column types**:Data types except for timestamp, binary, nchar and bool

**Applicable table types**:table, stable

**More explanations**:

- From version 2.1.3.0, function IRATE can be used on stble with `GROUP BY`, i.e. timelines generated by `GROUP BY tbname` on a stable.

### SUM

```
SELECT SUM(field_name) FROM tb_name [WHERE clause];
```

**Description**:The sum of a specific column in a table or stable

**Return value type**:Double precision floating number or long integer

**Applicable column types**:Data types except for timestamp, binary, nchar and bool

**Applicable table types**:table, stable

**Examples**:

```
taos> SELECT SUM(current), SUM(voltage), SUM(phase) FROM meters;
    sum(current)        |     sum(voltage)      |        sum(phase)         |
================================================================================
            103.200000763 |                  1984 |               2.640000001 |
Query OK, 1 row(s) in set (0.001702s)

taos> SELECT SUM(current), SUM(voltage), SUM(phase) FROM d1001;
    sum(current)        |     sum(voltage)      |        sum(phase)         |
================================================================================
            35.200000763 |                   658 |               0.950000018 |
Query OK, 1 row(s) in set (0.000980s)
```

### STDDEV

```
SELECT STDDEV(field_name) FROM tb_name [WHERE clause];
```

**Description**:Standard deviation of a specific column in a table or stable

**Return value type**:Double precision floating number

**Applicable column types**:Data types except for timestamp, binary, nchar and bool

**Applicable table types**:table, stable (starting from version 2.0.15.1)

**Examples**:

```
taos> SELECT STDDEV(current) FROM d1001;
    stddev(current)      |
============================
            1.020892909 |
Query OK, 1 row(s) in set (0.000915s)
```

### LEASTSQUARES

```
SELECT LEASTSQUARES(field_name, start_val, step_val) FROM tb_name [WHERE clause];
```

**Description**:统计表中某列的值是主键（时间戳）的拟合直线方程.start_val 是自变量初始值,step_val 是自变量的步长值.

**Return value type**:A string in the format of "(slope, intercept)"

**Applicable column types**:Data types except for timestamp, binary, nchar and bool

**Applicable table types**:table only

**Examples**:

```
taos> SELECT LEASTSQUARES(current, 1, 1) FROM d1001;
            leastsquares(current, 1, 1)             |
=====================================================
{slop:1.000000, intercept:9.733334}                 |
Query OK, 1 row(s) in set (0.000921s)
```

### MODE

```
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. It can't be used on timestamp column or tags.

**Return value type**:Same as the data type of the column being operated

**Applicable column types**:Data types except for timestamp

**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.

**Applicable version**:From version 2.6.0.0

**Examples**:

```
taos> select voltage from d002;
    voltage        |
========================
       1           |
       1           |
       2           |
       19          |
Query OK, 4 row(s) in set (0.003545s)

taos> select mode(voltage) from d002;
  mode(voltage)    |
========================
       1           |
Query OK, 1 row(s) in set (0.019393s)
```

### HYPERLOGLOG

```
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.

**Return value type**:Integer

**Applicable column types**:Any data type

**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.

**Applicable versions**:From version 2.6.0.0

**Examples**:

```
taos> select dbig from shll;
     dbig          |
========================
       1           |
       1           |
       1           |
       NULL        |
       2           |
       19          |
       NULL        |
       9           |
Query OK, 8 row(s) in set (0.003755s)

taos> select hyperloglog(dbig) from shll;
  hyperloglog(dbig)|
========================
       4           |
Query OK, 1 row(s) in set (0.008388s)
```

## Selection Functions

When any selective function is used, timestamp column or tag columns including `tbname` can be specified to show that the selected value are from which rows.

### MIN

```
SELECT MIN(field_name) FROM {tb_name | stb_name} [WHERE clause];
```

**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

**Applicable column types**:Data types except for timestamp, binary, nchar and bool

**Applicable table types**:table, stable

**Examples**:

```
taos> SELECT MIN(current), MIN(voltage) FROM meters;
    min(current)     | min(voltage) |
======================================
            10.20000 |          218 |
Query OK, 1 row(s) in set (0.001765s)

taos> SELECT MIN(current), MIN(voltage) FROM d1001;
    min(current)     | min(voltage) |
======================================
            10.30000 |          218 |
Query OK, 1 row(s) in set (0.000950s)
```

### MAX

```
SELECT MAX(field_name) FROM { tb_name | stb_name } [WHERE clause];
```

**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

**Applicable column types**:Data types except for timestamp, binary, nchar and bool

**Applicable table types**:table, stable

**Examples**:

```
taos> SELECT MAX(current), MAX(voltage) FROM meters;
    max(current)     | max(voltage) |
======================================
            13.40000 |          223 |
Query OK, 1 row(s) in set (0.001123s)

taos> SELECT MAX(current), MAX(voltage) FROM d1001;
    max(current)     | max(voltage) |
======================================
            12.60000 |          221 |
Query OK, 1 row(s) in set (0.000987s)
```

### FIRST

```
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

**Return value type**:Same as the column being operated

**Applicable column types**:Any data type

**Applicable table types**:table, stable

**More explanations**:

- 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
- No result will NOT be returned if all the columns in the result set are all NULL

**Examples**:

```
taos> SELECT FIRST(*) FROM meters;
        first(ts)        |    first(current)    | first(voltage) |     first(phase)     |
=========================================================================================
2018-10-03 14:38:04.000 |             10.20000 |            220 |              0.23000 |
Query OK, 1 row(s) in set (0.004767s)

taos> SELECT FIRST(current) FROM d1002;
    first(current)    |
=======================
            10.20000 |
Query OK, 1 row(s) in set (0.001023s)
```

### LAST

```
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

**Return value type**:Same as the column being operated

**Applicable column types**:Any data type

**Applicable table types**:table, stable

**More explanations**:

- 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.

**Examples**:

```
taos> SELECT LAST(*) FROM meters;
        last(ts)         |    last(current)     | last(voltage) |     last(phase)      |
========================================================================================
2018-10-03 14:38:16.800 |             12.30000 |           221 |              0.31000 |
Query OK, 1 row(s) in set (0.001452s)

taos> SELECT LAST(current) FROM d1002;
    last(current)     |
=======================
            10.30000 |
Query OK, 1 row(s) in set (0.000843s)
```

### TOP

```
SELECT TOP(field_name, K) FROM { tb_name | stb_name } [WHERE clause];
```

**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

**Applicable column types**:Data types except for timestamp, binary, nchar and bool

**Applicable table types**:table, stable

**More explanations**:

- _k_ must be in range [1,100]
- The timestamp associated with the selected values are returned too
- Can't be used with `FILL`

**Examples**:

```
taos> SELECT TOP(current, 3) FROM meters;
        ts            |   top(current, 3)    |
=================================================
2018-10-03 14:38:15.000 |             12.60000 |
2018-10-03 14:38:16.600 |             13.40000 |
2018-10-03 14:38:16.800 |             12.30000 |
Query OK, 3 row(s) in set (0.001548s)

taos> SELECT TOP(current, 2) FROM d1001;
        ts            |   top(current, 2)    |
=================================================
2018-10-03 14:38:15.000 |             12.60000 |
2018-10-03 14:38:16.800 |             12.30000 |
Query OK, 2 row(s) in set (0.000810s)
```

### BOTTOM

```
SELECT BOTTOM(field_name, K) FROM { tb_name | stb_name } [WHERE clause];
```

**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

**Applicable column types**: Data types except for timestamp, binary, nchar and bool

**Applicable table types**: table, stable

**More explanations**:

- _k_ must be in range [1,100]
- The timestamp associated with the selected values are returned too
- Can't be used with `FILL`

**Examples**:

```
taos> SELECT BOTTOM(voltage, 2) FROM meters;
        ts            | bottom(voltage, 2) |
===============================================
2018-10-03 14:38:15.000 |                218 |
2018-10-03 14:38:16.650 |                218 |
Query OK, 2 row(s) in set (0.001332s)

taos> SELECT BOTTOM(current, 2) FROM d1001;
        ts            |  bottom(current, 2)  |
=================================================
2018-10-03 14:38:05.000 |             10.30000 |
2018-10-03 14:38:16.800 |             12.30000 |
Query OK, 2 row(s) in set (0.000793s)
```

### PERCENTILE

```
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 precision floating point

**Applicable column types**: Data types except for timestamp, binary, nchar and bool

**Applicable table types**: table

**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.

**Examples**:

```
taos> SELECT PERCENTILE(current, 20) FROM d1001;
percentile(current, 20)  |
============================
            11.100000191 |
Query OK, 1 row(s) in set (0.000787s)
```

### APERCENTILE

```
SELECT APERCENTILE(field_name, P[, algo_type])
FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: Similar to `PERCENTILE`, but a simulated result is returned

**Return value type**: Double precision floating point

**Applicable column types**: Data types except for timestamp, binary, nchar and bool

**Applicable table types**: table, stable

**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.
- **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")`.
- When `t-digest` is used, `t-digest` sampling is used to calculate. It can be used from version 2.2.0.0.

**Nested query**: It can be used in both the outer query and inner query in a nested query.

```
taos> SELECT APERCENTILE(current, 20) FROM d1001;
apercentile(current, 20)  |
============================
            10.300000191 |
Query OK, 1 row(s) in set (0.000645s)

taos> select apercentile (count, 80, 'default') from stb1;
 apercentile (c0, 80, 'default') |
==================================
             601920857.210056424 |
Query OK, 1 row(s) in set (0.012363s)

taos> select apercentile (count, 80, 't-digest') from stb1;
 apercentile (c0, 80, 't-digest') |
===================================
              605869120.966666579 |
Query OK, 1 row(s) in set (0.011639s)
```

### LAST_ROW

```
SELECT LAST_ROW(field_name) FROM { tb_name | stb_name };
```

**Description**: The last row of a table or stable

**Return value type**: Same as the column being operated

**Applicable column types**: Any data type

**Applicable table types**: table, stable

**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.
- Can't be used with `INTERVAL`.

**Examples**:

```
 taos> SELECT LAST_ROW(current) FROM meters;
 last_row(current)   |
 =======================
             12.30000 |
 Query OK, 1 row(s) in set (0.001238s)

 taos> SELECT LAST_ROW(current) FROM d1002;
 last_row(current)   |
 =======================
             10.30000 |
 Query OK, 1 row(s) in set (0.001042s)
```

### INTERP [From version 2.3.1]

```
SELECT INTERP(field_name) FROM { tb_name | stb_name } [WHERE where_condition] [ RANGE(timestamp1,timestamp2) ] [EVERY(interval)] [FILL ({ VALUE | PREV | NULL | LINEAR | NEXT})];
```

**Description**: The value that matches the specified timestamp range is returned, if existing; or an interpolation value is returned.

**Return value type**: same as the column being operated

**Applicable column types**: Numeric data types

**Applicable table types**: table, stable, nested query

**More explanations**

- `INTERP` is used to get the value that matches the specified time slice from a column. If no such value exists an interpolation value will be returned based on `FILL` parameter.
- The input data of `INTERP` is the value of the specified column, `where` can be used to filter the original data. If no `where` condition is specified then all original data is the input.
- The output time range of `INTERP` is specified by `RANGE(timestamp1,timestamp2)` parameter, with timestamp1<=timestamp2. timestamp1 is the starting point of the output time range and must be specified. timestamp2 is the ending point of the output time range and must be specified. If `RANGE` is not specified, then the timestamp of the first row that matches the filter condition is treated as timestamp1, the timestamp of the last row that matches the filter condition is treated as timestamp2.
- The number of rows in the result set of `INTERP` is determined by the parameter `EVERY`. Starting from timestamp1, one interpolation is performed for every time interval specified `EVERY` parameter. If `EVERY` parameter is not used, the time windows will be considered as no ending timestamp, i.e. there is only one time window from timestamp1.
- Interpolation is performed based on `FILL` parameter. No interpolation is performed if `FILL` is not used, that means either the original data that matches is returned or nothing is returned.
- `INTERP` can only be used to interpolate in single timeline. So it must be used with `group by tbname` when it's used on a stable. It can't be used with `GROUP BY` when it's used in the inner query of a nested query.
- The result of `INTERP` is not influenced by `ORDER BY TIMESTAMP`, which impacts the output order only..

**Examples**: Based on the `meters` schema used throughout the documents

- Single point linear interpolation between "2017-07-14 18:40:00" and "2017-07-14 18:40:00:

```
 taos> SELECT INTERP(current) FROM t1 RANGE('2017-7-14 18:40:00','2017-7-14 18:40:00') FILL(LINEAR);
```

- Get an original data every 5 seconds, no interpolation, between "2017-07-14 18:00:00" and "2017-07-14 19:00:00:

```
 taos> SELECT INTERP(current) FROM t1 RANGE('2017-7-14 18:00:00','2017-7-14 19:00:00') EVERY(5s);
```

- Linear interpolation every 5 seconds between "2017-07-14 18:00:00" and "2017-07-14 19:00:00:

```
  taos> SELECT INTERP(current) FROM t1 RANGE('2017-7-14 18:00:00','2017-7-14 19:00:00') EVERY(5s) FILL(LINEAR);
```

- Backward interpolation every 5 seconds

```
  taos> SELECT INTERP(current) FROM t1 EVERY(5s) FILL(NEXT);
```

- Linear interpolation every 5 seconds between "2017-07-14 17:00:00" and "2017-07-14 20:00:00"

```
  taos> SELECT INTERP(current) FROM t1 where ts >= '2017-07-14 17:00:00' and ts <= '2017-07-14 20:00:00' RANGE('2017-7-14 18:00:00','2017-7-14 19:00:00') EVERY(5s) FILL(LINEAR);
```

### INTERP [Prior to version 2.3.1]

```
SELECT INTERP(field_name) FROM { tb_name | stb_name } WHERE ts='timestamp' [FILL ({ VALUE | PREV | NULL | LINEAR | NEXT})];
```

**Description**: The value of a specific column that matches the specified time slice

**Return value type**: Same as the column being operated

**Applicable column types**: Numeric data type

**Applicable table types**: table, stable

**More explanations**:

- It can be used from version 2.0.15.0
- Time slice must be specified. If there is no data matching the specified time slice, interpolation is performed based on `FILL` parameter. Conditions such as tags or `tbname` can be used `Where` clause can be used to filter data.
- The timestamp specified must be within the time range of the data rows of the table or stable. If it is beyond the valid time range, nothing is returned even with `FILL` parameter.
- `INTERP` can be used to query only single time point once. `INTERP` can be used with `EVERY` to get the interpolation value every time interval.
- **Examples**:

```
 taos> SELECT INTERP(*) FROM meters WHERE ts='2017-7-14 18:40:00.004';
        interp(ts)        |   interp(current)    | interp(voltage) |    interp(phase)     |
 ==========================================================================================
  2017-07-14 18:40:00.004 |              9.84020 |             216 |              0.32222 |
 Query OK, 1 row(s) in set (0.002652s)
```

If there is not any data corresponding to the specified timestamp, an interpolation value is returned if interpolation policy is specified by `FILL` parameter; or nothing is returned\

```
 taos> SELECT INTERP(*) FROM meters WHERE tbname IN ('d636') AND ts='2017-7-14 18:40:00.005';
 Query OK, 0 row(s) in set (0.004022s)

 taos> SELECT INTERP(*) FROM meters WHERE tbname IN ('d636') AND ts='2017-7-14 18:40:00.005' FILL(PREV);
        interp(ts)        |   interp(current)    | interp(voltage) |    interp(phase)     |
 ==========================================================================================
  2017-07-14 18:40:00.005 |              9.88150 |             217 |              0.32500 |
 Query OK, 1 row(s) in set (0.003056s)
```

Interpolation is performed every 5 milliseconds between `['2017-7-14 18:40:00', '2017-7-14 18:40:00.014']`

```
 taos> SELECT INTERP(current) FROM d636 WHERE ts>='2017-7-14 18:40:00' AND ts<='2017-7-14 18:40:00.014' EVERY(5a);
            ts            |   interp(current)    |
 =================================================
  2017-07-14 18:40:00.000 |             10.04179 |
  2017-07-14 18:40:00.010 |             10.16123 |
 Query OK, 2 row(s) in set (0.003487s)
```

### TAIL

```
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]

**Return value type**: Same as the column being operated

**Applicable column types**: Any data type except form timestamp, i.e. the primary key

**Applicable versions**: From version 2.6.0.0

**Examples**:

```
taos> select ts,dbig from tail2;
       ts            |         dbig          |
==================================================
2021-10-15 00:31:33.000 |                     1 |
2021-10-17 00:31:31.000 |                  NULL |
2021-12-24 00:31:34.000 |                     2 |
2022-01-01 08:00:05.000 |                    19 |
2022-01-01 08:00:06.000 |                  NULL |
2022-01-01 08:00:07.000 |                     9 |
Query OK, 6 row(s) in set (0.001952s)

taos> select tail(dbig,2,2) from tail2;
ts                      |    tail(dbig,2,2)     |
==================================================
2021-12-24 00:31:34.000 |                     2 |
2022-01-01 08:00:05.000 |                    19 |
Query OK, 2 row(s) in set (0.002307s)
```

### UNIQUE

```
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.

**Return value type**: Same as the column or tag being operated

**Applicable column types**: Any data types except for timestamp

**支持版本**: From version 2.6.0.0

**More explanations**:

- 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.

**Examples**:

```
taos> select ts,voltage from unique1;
       ts            |        voltage        |
==================================================
2021-10-17 00:31:31.000 |                     1 |
2022-01-24 00:31:31.000 |                     1 |
2021-10-17 00:31:31.000 |                     1 |
2021-12-24 00:31:31.000 |                     2 |
2022-01-01 08:00:01.000 |                    19 |
2021-10-17 00:31:31.000 |                  NULL |
2022-01-01 08:00:02.000 |                  NULL |
2022-01-01 08:00:03.000 |                     9 |
Query OK, 8 row(s) in set (0.003018s)

taos> select unique(voltage) from unique1;
ts                      |    unique(voltage)    |
==================================================
2021-10-17 00:31:31.000 |                     1 |
2021-10-17 00:31:31.000 |                  NULL |
2021-12-24 00:31:31.000 |                     2 |
2022-01-01 08:00:01.000 |                    19 |
2022-01-01 08:00:03.000 |                     9 |
Query OK, 5 row(s) in set (0.108458s)
```

## Scalar functions

### DIFF

```sql
SELECT {DIFF(field_name, ignore_negative) | DIFF(field_name)} FROM tb_name [WHERE clause];
```

**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

**Applicable column types**: Data types except for timestamp, binary, nchar and bool

**Applicable table types**: table, stable

**More explanations**:

- The number of result rows is the number of rows subtracted by one, no output for the first row
- From version 2.1.30, `DIFF` can be used on stable with `GROUP by tbname`
- From version 2.6.0, `ignore_negative` parameter is supported

**Examples**:

```sql
taos> SELECT DIFF(current) FROM d1001;
        ts            |    diff(current)     |
=================================================
2018-10-03 14:38:15.000 |              2.30000 |
2018-10-03 14:38:16.800 |             -0.30000 |
Query OK, 2 row(s) in set (0.001162s)
```

### DERIVATIVE

```
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` 参数指定, 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 precision floating point

**Applicable column types**: Data types except for timestamp, binary, nchar and bool

**Applicable table types**: table, stable

**More explanations**:

- It is available from version 2.1.3.0, the number of result rows is the number of total rows in the time range subtracted by one, no output for the first row.\
- It can be used together with `GROUP BY tbname` against a stable.

**Examples**:

```
taos> select derivative(current, 10m, 0) from t1;
           ts            | derivative(current, 10m, 0) |
========================================================
 2021-08-20 10:11:22.790 |                 0.500000000 |
 2021-08-20 11:11:22.791 |                 0.166666620 |
 2021-08-20 12:11:22.791 |                 0.000000000 |
 2021-08-20 13:11:22.792 |                 0.166666620 |
 2021-08-20 14:11:22.792 |                -0.666666667 |
Query OK, 5 row(s) in set (0.004883s)
```

### SPREAD

```
SELECT SPREAD(field_name) FROM { tb_name | stb_name } [WHERE clause];
```

**Description**: The difference between the max and the min of a specific column

**Return value type**: Double precision floating point

**Applicable column types**: Data types except for binary, nchar, and bool

**Applicable table types**: table, stable

**More explanations**: Can be used on a column of TIMESTAMP type, the result is the time range size.可

**Examples**:

```
taos> SELECT SPREAD(voltage) FROM meters;
    spread(voltage)      |
============================
            5.000000000 |
Query OK, 1 row(s) in set (0.001792s)

taos> SELECT SPREAD(voltage) FROM d1001;
    spread(voltage)      |
============================
            3.000000000 |
Query OK, 1 row(s) in set (0.000836s)
```

### CEIL

```
SELECT CEIL(field_name) FROM { tb_name | stb_name } [WHERE clause];
```

**Description**: The round up value of a specific column

**Return value type**: Same as the column being used

**Applicable data types**: Data types except for timestamp, binary, nchar, bool

**Applicable table types**: table, stable

**Applicable nested query**: inner query and outer query

**More explanations**:

- Can't be used on any tags of any type
- Arithmetic operation can be performed on the result of `ceil` function
- Can't be used with aggregate functions

### FLOOR

```
SELECT FLOOR(field_name) FROM { tb_name | stb_name } [WHERE clause];
```

**Description**: The round down value of a specific column

**More explanations**: The restrictions are same as `CEIL` function.

### ROUND

```
SELECT ROUND(field_name) FROM { tb_name | stb_name } [WHERE clause];
```

**Description**: The round value of a specific column.

**More explanations**: The restrictions are same as `CEIL` function.

### CSUM

    ```sql
    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**: Long integer for integers; Double for floating points. Timestamp is returned for each row.

**Applicable data types**: Data types except for timestamp, binary, nchar, and bool

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**More explanations**:

- Can't be used on tags when it's used on stable
- Arithmetic operation can't be performed on the result of `csum` function
- Can only be used with aggregate functions
- `Group by tbname` must be used together on a stable to force the result on a single timeline

**Applicable versions**: From 2.3.0.x

### MAVG

    ```sql
    SELECT MAVG(field_name, K) FROM { tb_name | stb_name } [WHERE clause]
    ```

**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 is _k_ is [1,1000].

**Return value type**: Double precision floating point

**Applicable data types**: Data types except for timestamp, binary, nchar, and bool

**Applicable nested query**: Inner query and Outer query

**Applicable table types**: table, stable

**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.\（Aggregation）函数一起使用；
- Must be used with `GROUP BY tbname` when it's used on a stable to force the result on each single timeline.该

**Applicable versions**: From 2.3.0.x

### SAMPLE

    ```sql
    SELECT SAMPLE(field_name, K) FROM { tb_name | stb_name } [WHERE clause]
    ```

**Description**: _k_ sampling values of a specific column. The applicable range of _k_ is [1,10000]

**Return value type**: Same as the column being operated plus the associated timestamp

**Applicable data types**: Any data type except for tags of stable

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**More explanations**:

- Arithmetic operation can't be operated on the result of `SAMPLE` function
- Must be used with `Group by tbname` when it's used on a stable to force the result on each single timeline

**Applicable versions**: From 2.3.0.x

### ASIN

```sql
SELECT ASIN(field_name) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: The anti-sine of a specific column

**Return value type**: Double if the input value is not NULL; or NULL if the input value is NULL

**Applicable data types**: Data types except for timestamp, binary, nchar, bool

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**:

- Can't be used with tags
- Can't be used with aggregate functions

### ACOS

```sql
SELECT ACOS(field_name) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: The anti-cosine of a specific column 获

**Return value type**: ouble if the input value is not NULL; or NULL if the input value is NULL

**Applicable data types**: Data types except for timestamp, binary, nchar, bool

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**:

- Can't be used with tags
- Can't be used with aggregate functions

### ATAN

```sql
SELECT ATAN(field_name) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: anti-tangent of a specific column

**Description**: The anti-cosine of a specific column 获

**Return value type**: Double if the input value is not NULL; or NULL if the input value is NULL

**Applicable data types**: Data types except for timestamp, binary, nchar, bool

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**:

- Can't be used with tags
- Can't be used with aggregate functions

### SIN

```sql
SELECT SIN(field_name) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: The sine of a specific column

**Description**: The anti-cosine of a specific column 获

**Return value type**: Double if the input value is not NULL; or NULL if the input value is NULL

**Applicable data types**: Data types except for timestamp, binary, nchar, bool

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**:

- Can't be used with tags
- Can't be used with aggregate functions

### COS

```sql
SELECT COS(field_name) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: The cosine of a specific column

**Description**: The anti-cosine of a specific column 获

**Return value type**: Double if the input value is not NULL; or NULL if the input value is NULL

**Applicable data types**: Data types except for timestamp, binary, nchar, bool

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**:

- Can't be used with tags
- Can't be used with aggregate functions

### TAN

```sql
SELECT TAN(field_name) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: The tangent of a specific column

**Description**: The anti-cosine of a specific column 获

**Return value type**: Double if the input value is not NULL; or NULL if the input value is NULL

**Applicable data types**: Data types except for timestamp, binary, nchar, bool

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**:

- Can't be used with tags
- Can't be used with aggregate functions

### POW

```sql
SELECT POW(field_name, power) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: The power of a specific column with `power` as the index

**Return value type**: Double if the input value is not NULL; or NULL if the input value is NULL

**Applicable data types**: Data types except for timestamp, binary, nchar, bool

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**:

- Can't be used with tags
- Can't be used with aggregate functions

### LOG

```sql
SELECT LOG(field_name, base) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: The log of a specific with `base` as the radix

**Return value type**: Double if the input value is not NULL; or NULL if the input value is NULL

**Applicable data types**: Data types except for timestamp, binary, nchar, bool

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**:

- Can't be used with tags
- Can't be used with aggregate functions

### ABS

```sql
SELECT ABS(field_name) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: The absolute of a specific column

**Return value type**: UBIGINT if the input value is integer; DOUBLE if the input value is FLOAT/DOUBLE 如

**Applicable data types**: Data types except for timestamp, binary, nchar, bool

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**:

- Can't be used with tags
- Can't be used with aggregate functions

### SQRT

```sql
SELECT SQRT(field_name) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: The square root of a specific column

**Return value type**: Double if the input value is not NULL; or NULL if the input value is NULL

**Applicable data types**: Data types except for timestamp, binary, nchar, bool

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**:

- Can't be used with tags
- Can't be used with aggregate functions

### CAST

```sql
SELECT CAST(expression AS type_name) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: It's used for type casting. The input parameter `expression` can be data columns, constants, scalar functions or arithmetic between them. Can't be used with tags, and can only be used in `select` clause.

**Return value type**: The type specified by parameter `type_name`

**Applicable data types**:

- Parameter `expression` can be any data type except for JSON, more specifically it can be any of BOOL/TINYINT/SMALLINT/INT/BIGINT/FLOAT/DOUBLE/BINARY(M)/TIMESTAMP/NCHAR(M)/TINYINT UNSIGNED/SMALLINT UNSIGNED/INT UNSIGNED/BIGINT UNSIGNED
- The output data type specified by `type_name` can only be one of BIGINT/BINARY(N)/TIMESTAMP/NCHAR(N)/BIGINT UNSIGNED

**Applicable versions**: From 2.6.0.0

**More explanations**:

- Error will be reported for unsupported type casting
- NULL will be returned if the input value is NULL
- Some values of some supported data types may not be casted, below are known issues:
  1）When casting BINARY/NCHAR to BIGINT/BIGINT UNSIGNED, some characters may be treated as illegal, for example "a" may be converted to 0.
  2）There may be overflow when casting singed integer or TIMESTAMP to unsigned BIGINT
  3）There may be overflow when casting unsigned BIGINT to BIGINT
  4）There may be overflow when casting FLOAT/DOUBLE to BIGINT or UNSIGNED BIGINT

### CONCAT

```sql
SELECT CONCAT(str1|column1, str2|column2, ...) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: The concatenation result of two or more strings, the number of strings to be concatenated is at least 2 and at most 8

**Return value type**: Same as the columns being operated, BINARY or NCHAR; or NULL if all the input are NULL

**Applicable data types**: The input data must be in either all BINARY or in all NCHAR; can't be used on tag columns

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

### CONCAT_WS

```
SELECT CONCAT_WS(separator, str1|column1, str2|column2, ...) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: The concatenation result of two or more strings with separator, the number of strings to be concatenated is at least 3 and at most 9

**Return value type**: Same as the columns being operated, BINARY or NCHAR; or NULL if all the input are NULL

**Applicable data types**: The input data must be in either all BINARY or in all NCHAR; can't be used on tag columns

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**:

- If the value of `separator` is NULL, the output is NULL. If the value of `separator` is not NULL but other input are all NULL, the output is empty string.

### LENGTH

```
SELECT LENGTH(str|column) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: The length in bytes of a string

**Return value type**: Integer

**Applicable data types**: BINARY or NCHAR, can't be used on tags

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**

- If the input value is NULL, the output is NULL too

### CHAR_LENGTH

```
SELECT CHAR_LENGTH(str|column) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: The length in number of characters of a string

**Return value type**: Integer

**Applicable data types**: BINARY or NCHAR, can't be used on tags

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**

- If the input value is NULL, the output is NULL too

### LOWER

```
SELECT LOWER(str|column) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: Convert the input string to lower case

**Return value type**: Same as input

**Applicable data types**: BINARY or NCHAR, can't be used on tags

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**

- If the input value is NULL, the output is NULL too

### UPPER

```
SELECT UPPER(str|column) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: Convert the input string to upper case

**Return value type**: Same as input

**Applicable data types**: BINARY or NCHAR, can't be used on tags

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**

- If the input value is NULL, the output is NULL too

### LTRIM

```
SELECT LTRIM(str|column) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: Remove the left leading blanks of a string

**Return value type**: Same as input

**Applicable data types**: BINARY or NCHAR, can't be used on tags

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**

- If the input value is NULL, the output is NULL too

### RTRIM

```
SELECT RTRIM(str|column) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: Remove the right tailing blanks of a string

**Return value type**: Same as input

**Applicable data types**: BINARY or NCHAR, can't be used on tags

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**

- If the input value is NULL, the output is NULL too

### SUBSTR

```
SELECT SUBSTR(str,pos[,len]) FROM { tb_name | stb_name } [WHERE clause]
```

**Description**: The sub-string starting from `pos` with length of `len` from the original string `str`

**Return value type**: Same as input

**Applicable data types**: BINARY or NCHAR, can't be used on tags

**Applicable table types**: table, stable

**Applicable nested query**: Inner query and Outer query

**Applicable versions**: From 2.6.0.0

**More explanations**:

- 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.

### Arithmetic Operations

```
SELECT field_name [+|-|*|/|%][Value|field_name] FROM { tb_name | stb_name }  [WHERE clause];
```

**Description**: The sum, difference, product, quotient, or remainder between one or more columns

**Return value type**: Double precision floating point

**Applicable column types**: Data types except for timestamp, binary, nchar, bool

**Applicable table types**: table, stable

**More explanations**:

- Arithmetic operations can be performed on two or more columns, `()` can be used to control the precedence
- NULL doesn't participate the operation, if one of the operands is NULL then result is NULL

**Examples**:

```
taos> SELECT current + voltage * phase FROM d1001;
(current+(voltage*phase)) |
============================
            78.190000713 |
            84.540003240 |
            80.810000718 |
Query OK, 3 row(s) in set (0.001046s)
```

### STATECOUNT

```
SELECT STATECOUNT(field_name, oper, val) FROM { tb_name | stb_name } [WHERE clause];
```

**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), the value is case insensitive
- val ： Numeric types

**Return value type**: Integer

**Applicable data types**: Data types excpet for timestamp, binary, nchar, bool

**Applicable table types**: table, stable

**Applicable nested query**: Outer query only

**Applicable versions**: From 2.6.0.0

**More explanations**:

- Must be used together with `GROUP 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

**Examples**:

```
taos> select ts,dbig from statef2;
          ts               |         dbig          |
========================================================
2021-10-15 00:31:33.000000000 |                     1 |
2021-10-17 00:31:31.000000000 |                  NULL |
2021-12-24 00:31:34.000000000 |                     2 |
2022-01-01 08:00:05.000000000 |                    19 |
2022-01-01 08:00:06.000000000 |                  NULL |
2022-01-01 08:00:07.000000000 |                     9 |
Query OK, 6 row(s) in set (0.002977s)

taos> select stateCount(dbig,GT,2) from statef2;
ts               |         dbig          | statecount(dbig,gt,2) |
================================================================================
2021-10-15 00:31:33.000000000 |                     1 |                    -1 |
2021-10-17 00:31:31.000000000 |                  NULL |                  NULL |
2021-12-24 00:31:34.000000000 |                     2 |                    -1 |
2022-01-01 08:00:05.000000000 |                    19 |                     1 |
2022-01-01 08:00:06.000000000 |                  NULL |                  NULL |
2022-01-01 08:00:07.000000000 |                     9 |                     2 |
Query OK, 6 row(s) in set (0.002791s)
```

### STATEDURATION

```
SELECT stateDuration(field_name, oper, val, unit) FROM { tb_name | stb_name } [WHERE clause];
```

**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), the value is case insensitive
- val ： Numeric types
- unit: The unit of time interval, can be [1s, 1m, 1h], default is 1s

**Return value type**: Integer

**Applicable data types**: Data types excpet for timestamp, binary, nchar, bool

**Applicable table types**: table, stable

**Applicable nested query**: Outer query only

**Applicable versions**: From 2.6.0.0

**More explanations**:

- Must be used together with `GROUP 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

**Examples**:

```
taos> select ts,dbig from statef2;
          ts               |         dbig          |
========================================================
2021-10-15 00:31:33.000000000 |                     1 |
2021-10-17 00:31:31.000000000 |                  NULL |
2021-12-24 00:31:34.000000000 |                     2 |
2022-01-01 08:00:05.000000000 |                    19 |
2022-01-01 08:00:06.000000000 |                  NULL |
2022-01-01 08:00:07.000000000 |                     9 |
Query OK, 6 row(s) in set (0.002407s)

taos> select stateDuration(dbig,GT,2) from statef2;
ts               |         dbig          | stateduration(dbig,gt,2) |
===================================================================================
2021-10-15 00:31:33.000000000 |                     1 |                       -1 |
2021-10-17 00:31:31.000000000 |                  NULL |                     NULL |
2021-12-24 00:31:34.000000000 |                     2 |                       -1 |
2022-01-01 08:00:05.000000000 |                    19 |                        0 |
2022-01-01 08:00:06.000000000 |                  NULL |                     NULL |
2022-01-01 08:00:07.000000000 |                     9 |                        2 |
Query OK, 6 row(s) in set (0.002613s)
```

## Time Functions

From version 2.6.0.0, below time related functions can be used in TDengine.

### NOW

```sql
SELECT NOW() FROM { tb_name | stb_name } [WHERE clause];
SELECT select_expr FROM { tb_name | stb_name } WHERE ts_col cond_operatior NOW();
INSERT INTO tb_name VALUES (NOW(), ...);
```

**Description**: The current time of the client side system

**Return value type**: TIMESTAMP

**Applicable column types**: TIMESTAMP only

**Applicable table types**: table, stable

**More explanations**:

- Add and Subtract operation can be performed, for example NOW() + 1s, the time unit can be:
  b(nanosecond), u(microsecond), a(millisecond)), s(second), m(minute), h(hour), d(day), w(week)
- The precision of the returned timestamp is same as the precision set for the current data base in use

**Examples**:

```sql
taos> SELECT NOW() FROM meters;
          now()          |
==========================
  2022-02-02 02:02:02.456 |
Query OK, 1 row(s) in set (0.002093s)

taos> SELECT NOW() + 1h FROM meters;
       now() + 1h         |
==========================
  2022-02-02 03:02:02.456 |
Query OK, 1 row(s) in set (0.002093s)

taos> SELECT COUNT(voltage) FROM d1001 WHERE ts < NOW();
        count(voltage)       |
=============================
                           5 |
Query OK, 5 row(s) in set (0.004475s)

taos> INSERT INTO d1001 VALUES (NOW(), 10.2, 219, 0.32);
Query OK, 1 of 1 row(s) in database (0.002210s)
```

### TODAY

```sql
SELECT TODAY() FROM { tb_name | stb_name } [WHERE clause];
SELECT select_expr FROM { tb_name | stb_name } WHERE ts_col cond_operatior TODAY()];
INSERT INTO tb_name VALUES (TODAY(), ...);
```

**Description**: The timestamp of 00:00:00 of the client side system

**Return value type**: TIMESTAMP

**Applicable column types**: TIMESTAMP only

**Applicable table types**: table, stable

**More explanations**:

- Add and Subtract operation can be performed, for example NOW() + 1s, the time unit can be:
  b(nanosecond), u(microsecond), a(millisecond)), s(second), m(minute), h(hour), d(day), w(week)
- The precision of the returned timestamp is same as the precision set for the current data base in use

**Examples**:

```sql
taos> SELECT TODAY() FROM meters;
         today()          |
==========================
  2022-02-02 00:00:00.000 |
Query OK, 1 row(s) in set (0.002093s)

taos> SELECT TODAY() + 1h FROM meters;
      today() + 1h        |
==========================
  2022-02-02 01:00:00.000 |
Query OK, 1 row(s) in set (0.002093s)

taos> SELECT COUNT(voltage) FROM d1001 WHERE ts < TODAY();
        count(voltage)       |
=============================
                           5 |
Query OK, 5 row(s) in set (0.004475s)

taos> INSERT INTO d1001 VALUES (TODAY(), 10.2, 219, 0.32);
Query OK, 1 of 1 row(s) in database (0.002210s)
```

### TIMEZONE

```sql
SELECT TIMEZONE() FROM { tb_name | stb_name } [WHERE clause];
```

**Description**: The timezone of the client side system

**Return value type**: BINARY

**Applicable column types**: None

**Applicable table types**: table, stable

**Examples**:

```sql
taos> SELECT TIMEZONE() FROM meters;
           timezone()           |
=================================
 UTC (UTC, +0000)               |
Query OK, 1 row(s) in set (0.002093s)
```

### TO_ISO8601

```sql
SELECT TO_ISO8601(ts_val | ts_col) FROM { tb_name | stb_name } [WHERE clause];
```

**Description**: The ISO8601 date/time format converted from a UNIX timestamp, plus the timezone of the client side system

**Return value type**: BINARY

**Applicable column types**: TIMESTAMP, constant or a column

**Applicable table types**: table, stable

**More explanations**:

- If the input is UNIX timestamp constant, 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

**Examples**:

```sql
taos> SELECT TO_ISO8601(1643738400) FROM meters;
   to_iso8601(1643738400)    |
==============================
 2022-02-02T02:00:00+0800    |

taos> SELECT TO_ISO8601(ts) FROM meters;
       to_iso8601(ts)        |
==============================
 2022-02-02T02:00:00+0800    |
 2022-02-02T02:00:00+0800    |
 2022-02-02T02:00:00+0800    |
```

### TO_UNIXTIMESTAMP

```sql
SELECT TO_UNIXTIMESTAMP(datetime_string | ts_col) FROM { tb_name | stb_name } [WHERE clause];
```

**Description**: UNIX timestamp converted from a string of date/time format

**Return value type**: Long integer

**Applicable column types**: Constant or column of BINARY/NCHAR

**Applicable table types**: table, stable

**More explanations**:

- The input string must be compatible with ISO8601/RFC3339 standard, 0 will be returned if the string can't be covnerted
- The precision of the returned timestamp is same as the precision set for the current data base in use

**Examples**:

```sql
taos> SELECT TO_UNIXTIMESTAMP("2022-02-02T02:00:00.000Z") FROM meters;
to_unixtimestamp("2022-02-02T02:00:00.000Z") |
==============================================
                               1643767200000 |

taos> SELECT TO_UNIXTIMESTAMP(col_binary) FROM meters;
      to_unixtimestamp(col_binary)     |
========================================
                         1643767200000 |
                         1643767200000 |
                         1643767200000 |
```

### TIMETRUNCATE

```sql
SELECT TIMETRUNCATE(ts_val | datetime_string | ts_col, time_unit) FROM { tb_name | stb_name } [WHERE clause];
```

**Description**: Truncate the input timestamp with unit specified by `time_unit`\

**Return value type**: TIMESTAMP\

**Applicable column types**: UNIX timestamp constant, string constant of date/time format, or a column of timestamp

**Applicable table types**: table, stable

**More explanations**:

- Time unit specified by `time_unit` can be:
  1u(microsecond),1a(millisecond),1s(second),1m(minute),1h(hour),1d(day).
- The precision of the returned timestamp is same as the precision set for the current data base in use

**Examples**:

```sql
taos> SELECT TIMETRUNCATE(1643738522000, 1h) FROM meters;
  timetruncate(1643738522000, 1h) |
===================================
      2022-02-02 02:00:00.000     |
Query OK, 1 row(s) in set (0.001499s)

taos> SELECT TIMETRUNCATE("2022-02-02 02:02:02", 1h) FROM meters;
  timetruncate("2022-02-02 02:02:02", 1h) |
===========================================
      2022-02-02 02:00:00.000             |
Query OK, 1 row(s) in set (0.003903s)

taos> SELECT TIMETRUNCATE(ts, 1h) FROM meters;
  timetruncate(ts, 1h)   |
==========================
 2022-02-02 02:00:00.000 |
 2022-02-02 02:00:00.000 |
 2022-02-02 02:00:00.000 |
Query OK, 3 row(s) in set (0.003903s)
```

### TIMEDIFF

```sql
SELECT TIMEDIFF(ts_val1 | datetime_string1 | ts_col1, ts_val2 | datetime_string2 | ts_col2 [, time_unit]) FROM { tb_name | stb_name } [WHERE clause];
```

**Description**: The difference between two timestamps, and rounded to the time unit specified by `time_unit`

**Return value type**: Long Integer

**Applicable column types**: UNIX timestamp constant, string constant of date/time format, or a column of TIMESTAMP type

**Applicable table types**: table, stable

**More explanations**:

- Time unit specified by `time_unit` can be:
  1u(microsecond),1a(millisecond),1s(second),1m(minute),1h(hour),1d(day).
- The precision of the returned timestamp is same as the precision set for the current data base in use

**Examples**:

```sql
taos> SELECT TIMEDIFF(1643738400000, 1643742000000) FROM meters;
 timediff(1643738400000, 1643742000000) |
=========================================
                                3600000 |
Query OK, 1 row(s) in set (0.002553s)
taos> SELECT TIMEDIFF(1643738400000, 1643742000000, 1h) FROM meters;
 timediff(1643738400000, 1643742000000, 1h) |
=============================================
                                          1 |
Query OK, 1 row(s) in set (0.003726s)

taos> SELECT TIMEDIFF("2022-02-02 03:00:00", "2022-02-02 02:00:00", 1h) FROM meters;
 timediff("2022-02-02 03:00:00", "2022-02-02 02:00:00", 1h) |
=============================================================
                                                          1 |
Query OK, 1 row(s) in set (0.001937s)

taos> SELECT TIMEDIFF(ts_col1, ts_col2, 1h) FROM meters;
   timediff(ts_col1, ts_col2, 1h) |
===================================
                                1 |
Query OK, 1 row(s) in set (0.001937s)
```