Project: TDengine (App Definition and Development / Database)
File: 10-function.md

title: Functions sidebar_label: Functions description: This document describes the standard SQL functions available in TDengine. tocmaxheading_level: 4 Scalar functions return one result for each row. ```sql ABS(expr) ``` Description: The absolute value of a specific field. Return value type: Same as the field being used Applicable data types: Numeric 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 Usage: This function can only be used on data columns. It can be used with selection and projection functions but not with aggregation functions. ```sql ACOS(expr) ``` Description: The arc cosine of a specific field. Return value type: Double Applicable data types: Numeric 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 Usage: This function can only be used on data columns. It can be used with selection and projection functions but not with aggregation functions. ```sql ASIN(expr) ``` Description: The arc sine of a specific field. Return value type: Double Applicable data types: Numeric 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 Usage: This function can only be used on data columns. It can be used with selection and projection functions but not with aggregation functions. ```sql ATAN(expr) ``` Description: The arc tangent of a specific field. Return value type: Double Applicable data types: Numeric 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 Usage: This function can only be used on data columns. It can be used with selection and projection functions but not with aggregation functions. ```sql CEIL(expr) ``` Description: The rounded up value of a specific field Return value type: Same as the field being used Applicable data types: Numeric 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. Usage: This function can only be used on data columns. It can be used with selection and projection functions but not with aggregation functions. ```sql COS(expr) ``` Description: The cosine of a specific field. Return value type: Double Applicable data types: Numeric 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 Usage: This function can only be used on data columns. It can be used with selection and projection functions but not with aggregation functions. ```sql FLOOR(expr) ``` Description: The rounded down value of a specific field More explanations: The restrictions are same as those of the `CEIL` function. ```sql LOG(expr [, base]) ``` 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 Applicable data types: Numeric 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 Usage: This function can only be used on data columns. It can be used with selection and projection functions but not with aggregation functions. ```sql POW(expr, power) ``` Description: The power of a specific field with `power` as the

Return value type: Double Applicable data types: Numeric 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 Usage: This function can only be used on data columns. It can be used with selection and projection functions but not with aggregation functions. ```sql ROUND(expr) ``` Description: The rounded value of a specific field. More explanations: The restrictions are same as those of the `CEIL` function. ```sql SIN(expr) ``` Description: The sine of a specific field. Return value type: Double Applicable data types: Numeric 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 Usage: This function can only be used on data columns. It can be used with selection and projection functions but not with aggregation functions. ```sql SQRT(expr) ``` Description: The square root of a specific field. Return value type: Double Applicable data types: Numeric 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 Usage: This function can only be used on data columns. It can be used with selection and projection functions but not with aggregation functions. ```sql TAN(expr) ``` Description: The tangent of a specific field. Return value type: Double Applicable data types: Numeric 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 Usage: This function can only be used on data columns. It can be used with selection and projection functions but not with aggregation functions. Concatenation functions take strings as input and produce string or numeric values as output. ```sql CHAR_LENGTH(expr) ``` Description: The length in number of characters of a string Return value type: Bigint Applicable data types: VARCHAR and NCHAR 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 ```sql CONCAT(expr1, expr2 [, expr] ...) ``` Description: The concatenation result of two or more strings 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 and NCHAR You can concatenate between 2 and 8 strings. 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 ```sql CONCATWS(separatorexpr, expr1, expr2 [, expr] ...) ``` Description: The concatenation result of two or more strings with separator 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 and NCHAR You can concatenate between 3 and 9 strings. 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 ```sql LENGTH(expr) ``` Description: The length in bytes Return value type: Bigint Applicable data types: VARCHAR and NCHAR and VARBINARY Nested query: It can be used in both the outer query and inner query in a nested

Applicable table types: standard tables and supertables ```sql LOWER(expr) ``` Description: Convert the input string to lower case Return value type: Same as input Applicable data types: VARCHAR and NCHAR 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 ```sql LTRIM(expr) ``` Description: Remove the left leading blanks of a string Return value type: Same as input Applicable data types: VARCHAR and NCHAR 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 ```sql LTRIM(expr) ``` Description: Remove the right tailing blanks of a string Return value type: Same as input Applicable data types: VARCHAR and NCHAR 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 ```sql SUBSTR(expr, pos [, len]) ``` 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. Return value type: Same as input 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. Nested query: It can be used in both the outer query and inner query in a nested query. Applicable table types: table, STable ```sql UPPER(expr) ``` Description: Convert the input string to upper case Return value type: Same as input Applicable data types: VARCHAR and NCHAR Nested query: It can be used in both the outer query and inner query in a nested query. Applicable table types: table, STable Conversion functions change the data type of a value. ```sql CAST(expr AS type_name) ``` Description: Convert the input data `expr` 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` Applicable data types: All data types except JSON and VARBINARY. If type_name is VARBINARY, expr can only be VARCHAR. 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: Error will be reported for unsupported type casting Some values of some supported data types may not be casted, below are known issues: 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. 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. 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. ```sql TO_ISO8601(expr [, timezone]) ``` 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 Applicable data types: Integers and timestamps 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: You can specify a time zone in the following format: [z/Z, +/-hhmm, +/-hh, +/-hh:mm]. For example, TO_ISO8601(1,

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 ```sql TOJSON(strliteral) ``` Description: Converts a string into JSON. Return value type: JSON 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. Nested query: It can be used in both the outer query and inner query in a nested query. Applicable table types: table, STable ```sql TOUNIXTIMESTAMP(expr [, returntimestamp]) return_timestamp: { 0 | 1 } ``` Description: UNIX timestamp converted from a string of date/time format Return value type: BIGINT, TIMESTAMP Applicable column types: VARCHAR and NCHAR 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 can't be converted The precision of the returned timestamp is same as the precision set for the current data base in use return_timestamp indicates whether the returned value type is TIMESTAMP or not. If this parameter set to 1, function will return TIMESTAMP type. Otherwise function will return BIGINT type. If parameter is omitted, default return value type is BIGINT. ```sql TOCHAR(ts, formatstr_literal) ``` Description: Convert a ts column to string as the format specified Version: Since ver-3.2.2.0 Return value type: VARCHAR Applicable column types: TIMESTAMP 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 Supported Formats | Format | Comment | example | | - | - | - | | AM,am,PM,pm | Meridiem indicator(without periods) | 07:00:00am | | A.M.,a.m.,P.M.,p.m. | Meridiem indicator(with periods) |

| | YYYY,yyyy | year, 4 or more digits | 2023-10-10 | | YYY,yyy | year, last 3 digits | 023-10-10 | | YY,yy | year, last 2 digits | 23-10-10 | | Y,y | year, last digit | 3-10-10 | | MONTH | full uppercase of month | 2023-JANUARY-01 | | Month | full capitalized month | 2023-January-01 | | month | full lowercase of month | 2023-january-01 | | MON | abbreviated uppercase of month(3 char) | JAN, SEP | | Mon | abbreviated capitalized month | Jan, Sep | | mon | abbreviated lowercase of month | jan, sep | | MM,mm | month number 01-12 | 2023-01-01 | | DD,dd | month day, 01-31 | | | DAY | full uppercase of week day | MONDAY | | Day | full capitalized week day | Monday | | day | full lowercase of week day | monday | | DY | abbreviated uppercase of week day | MON | | Dy | abbreviated capitalized week day | Mon | | dy | abbreviated lowercase of week day | mon | | DDD | year day, 001-366 | | | D,d | week day number, 1-7, Sunday(1) to Saturday(7) | | | HH24,hh24 | hour of day, 00-23 | 2023-01-30 23:59:59 | | hh12,HH12, hh, HH | hour of day, 01-12 | 2023-01-30 12:59:59PM | | MI,mi | minute, 00-59 | | | SS,ss | second, 00-59 | | | MS,ms | milli second, 000-999 | | | US,us | micro second, 000000-999999 | | | NS,ns | nano second, 000000000-999999999 | | | TZH,tzh | time zone hour | 2023-01-30 11:59:59PM +08 | More explanations: The output format of `Month`, `Day` are left aligined, like`2023-OCTOBER -01`, `2023-SEPTEMBER-01`, `September` is the longest, no paddings. Week days are slimilar. When `ms`,`us`,`ns` are used in `tochar`, like `tochar(ts, 'yyyy-mm-dd hh:mi:ss.ms.us.ns')`, The time of `ms`,`us`,`ns` corresponds to the same fraction seconds. When ts is `1697182085123`, the output of `ms` is `123`, `us` is `123000`, `ns` is `123000000`. If we want to output some characters of format without converting, surround it with double quotes. `tochar(ts, 'yyyy-mm-dd "is formated by yyyy-mm-dd"')`. If want to output double quotes, add a back slash before double quote, like `tochar(ts, '\"yyyy-mm-dd\"')` will output `"2023-10-10"`. For formats that output digits, the uppercase and lowercase formats are the same. It's recommended to put time zone in the format, if not, the default time zone will be that in server or client. The precision of the input timestamp will be recognized automatically according to the precision of the table used, milliseconds will be used if no table is specified. ```sql TOTIMESTAMP(tsstrliteral, formatstr_literal) ``` Description: Convert a formated timestamp string to a timestamp Version: Since ver-3.2.2.0 Return value type: TIMESTAMP Applicable column types: VARCHAR 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 Supported Formats: The same as `TO_CHAR`. More explanations: When `ms`, `us`, `ns` are used in `totimestamp`, if multi of them are specified, the results are accumulated. For example, `totimestamp('2023-10-10 10:10:10.123.000456.000000789', 'yyyy-mm-dd hh:mi:ss.ms.us.ns')` will output the timestamp of `2023-10-10 10:10:10.123456789`. The uppercase or lowercase of `MONTH`, `MON`, `DAY`, `DY` and formtas that output digits have same effect when used in `totimestamp`, like `totimestamp('2023-JANUARY-01', 'YYYY-month-dd')`, `month` can be replaced by `MONTH`, or `month`. The cases are ignored. If multi times are specified for one component, the previous will be overwritten. Like `to_timestamp('2023-22-10-10', 'yyyy-yy-MM-dd')`, the output year will be `2022`. To avoid unexpected time zone used during the convertion, it's recommended to put time zone in the ts string, e.g. '2023-10-10 10:10:10+08'. If time zone not specified, default will be that in server or client. The default timestamp if some components are not specified will be: `1970-01-01 00:00:00` with the timezone specified or default to local timezone. Only `DDD` is specified without `DD` is not supported currently, e.g. format 'yyyy-mm-ddd' is not supported, but 'yyyy-mm-dd' is supported. If `AM` or `PM` is specified in formats, the Hour must between `1-12`. In some cases, `totimestamp` can convert correctly even the format and the timestamp string are not totally matched. Like `totimetamp('200101/2', 'yyyyMM1/dd')`, the digit `1` in format string are ignored, and the output timestsamp is `2001-01-02 00:00:00`. Spaces and tabs in formats and tiemstamp string are also ignored automatically. The precision of the output timestamp will be the same as the table in SELECT stmt, millisecond will be used if no table is specified. The output of `select totimestamp('2023-08-1 10:10:10.123456789', 'yyyy-mm-dd hh:mi:ss.ns')` will be truncated to millisecond precision. If a nano precision table is specified, no truncation will be applied. Like `select totimestamp('2023-08-1 10:10:10.123456789', 'yyyy-mm-dd hh:mi:ss.ns') from dbns.tablens limit 1`. These functions perform operations on times and dates. All functions that return the current time, such as `NOW`, `TODAY`, and `TIMEZONE`, are calculated only once per statement even if they appear multiple

```sql NOW() ``` Description: The current time of the client side system Return value type: TIMESTAMP Applicable column types: TIMESTAMP only 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: 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 ```sql TIMEDIFF(expr1, expr2 [, time_unit]) ``` Description: The difference between two timestamps, and rounded to the time unit specified by `time_unit` Return value type: BIGINT Applicable column types: UNIX-style timestamps in BIGINT and TIMESTAMP format and other timestamps in VARCHAR and NCHAR format 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: Time unit specified by `time_unit` can be: 1b (nanoseconds), 1u (microseconds), 1a (milliseconds), 1s (seconds), 1m (minutes), 1h (hours), 1d (days), or 1w (weeks) The precision of the returned timestamp is same as the precision set for the current data base in use If the input data is not formatted as a timestamp, the returned value is null. ```sql TIMETRUNCATE(expr, timeunit [, usecurrent_timezone]) usecurrenttimezone: { 0 | 1 } ``` Description: Truncate the input timestamp with unit specified by `time_unit` Return value type: TIMESTAMP Applicable column types: UNIX-style timestamps in BIGINT and TIMESTAMP format and other timestamps in VARCHAR and NCHAR format Applicable table types: standard tables and supertables More explanations: Time unit specified by `time_unit` can be: 1b (nanoseconds), 1u (microseconds), 1a (milliseconds), 1s (seconds), 1m (minutes), 1h (hours), 1d (days), or 1w (weeks) The precision of the returned timestamp is same as the precision set for the current data base in use If the input data is not formatted as a timestamp, the returned value is null. When using 1d/1w as the time unit to truncate timestamp, you can specify whether to truncate based on the current time zone by setting the usecurrenttimezone parameter. Value 0 indicates truncation using the UTC time zone, value 1 indicates truncation using the current time zone. For example, if the time zone configured by the Client is UTC + 0800, TIMETRUNCATE ('2020-01-01 23:00:00', 1d, 0) returns the result of '2020-01-01 08:00:00'. When using TIMETRUNCATE ('2020-01-01 23:00:00', 1d, 1), the result is 2020-01-01 00:00:00 '. When usecurrenttimezone is not specified, usecurrenttimezone defaults to 1. ```sql TIMEZONE() ``` Description: The timezone of the client side system Applicable data types: VARCHAR Applicable column types: None Applicable table types: standard tables and supertables ```sql 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: standard tables and supertables More explanations: Add and Subtract operation can be performed, for example TODAY() + 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 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: ```sql APERCENTILE(expr, p [, algo_type]) algo_type: { "default" | "t-digest" } ``` Description: Similar to `PERCENTILE`, but a simulated result is returned Return value type: DOUBLE Applicable data types: Numeric 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

`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. The approximation result of `t-digest` algorithm is sensitive to input data order. For example, when querying STable with different input data order there might be minor differences in calculated results. ```sql AVG(expr) ``` Description: The average value of the specified fields. Return value type: DOUBLE Applicable data types: Numeric Applicable table types: standard tables and supertables ```sql COUNT({* | expr}) ``` Description: The number of records in the specified fields. Return value type: BIGINT Applicable data types: Numeric Applicable table types: standard tables and supertables More explanation: Wildcard (\*) is used to represent all columns. If you input a specific column, the number of non-null values in the column is returned. ```sql ELAPSED(tsprimarykey [, 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 calculated time length within each time window. If it's used without `INTERVAL` clause, 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 if the input value is not NULL; Return value type: TIMESTAMP Applicable tables: table, STable, outer in nested query Explanations: `tsprimarykey` parameter can only be the first column of a table, i.e. timestamp primary key. The minimum value of `timeunit` is the time precision of the database. If `timeunit` is not specified, the time precision of the database is used as the default time unit. Time unit specified by `time_unit` can be: 1b (nanoseconds), 1u (microseconds), 1a (milliseconds), 1s (seconds), 1m (minutes), 1h (hours), 1d (days), or 1w (weeks) 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. 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`. ```sql LEASTSQUARES(expr, startval, stepval) ``` Description: The linear regression function of a specified column, `startval` is the initial value and `stepval` is the step

Return value type: A string in the format of "(slope, intercept)" Applicable data types: Numeric Applicable table types: table only ```sql SPREAD(expr) ``` Description: The difference between the max and the min of a specific column Return value type: DOUBLE Applicable data types: Integers and timestamps Applicable table types: standard tables and supertables ```sql STDDEV(expr) ``` Description: Standard deviation of a specific column in a table or STable Return value type: DOUBLE Applicable data types: Numeric Applicable table types: standard tables and supertables ```sql SUM(expr) ``` Description: The sum of a specific column in a table or STable Return value type: DOUBLE or BIGINT Applicable data types: Numeric Applicable table types: standard tables and supertables ```sql HYPERLOGLOG(expr) ``` 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 recommended to use `select count(data) from (select unique(col) as data from table)` in this case. Return value type: Integer Applicable data types: Numeric Applicable table types: standard tables and supertables ```sql HISTOGRAM(expr, bintype, bindescription, normalized) ``` Description: Returns count of data points in user-specified ranges. Return value type If normalized is set to 1, a DOUBLE is returned; otherwise a BIGINT is returned Applicable data types: Numeric Applicable table types: table, STable Explanations: bintype: parameter to indicate the bucket type, valid inputs are: "userinput", "linearbin", "logbin". bindescription: parameter to describe how to generate buckets can be in the following JSON formats for each bintype respectively: "user_input": "[1, 3, 5, 7]": User specified bin values. "linear_bin": "&lcub;"start": 0.0, "width": 5.0, "count": 5, "infinity": true&rcub;" "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]. "log_bin": "&lcub;"start":1.0, "factor": 2.0, "count": 5, "infinity": true&rcub;" "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. ```sql PERCENTILE(expr, p [, p1] ...) ``` 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: This function takes 2 minimum and 11 maximum parameters, and it can simultaneously return 10 percentiles at most. If 2 parameters are given, a single percentile is returned and the value type is DOUBLE. If more than 2 parameters are given, the return value type is a VARCHAR string, the format of which is a JSON ARRAY containing all return values. Applicable column types: Numeric 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. When calculating multiple percentiles of a specific column, a single PERCENTILE function with multiple parameters is advised, as this can largely reduce the query response time. For example, using SELECT percentile(col, 90, 95, 99) FROM table will perform better than SELECT percentile(col, 90), percentile(col, 95), percentile(col, 99) from table. 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. ```sql BOTTOM(expr, k) ``` Description: The least k values of a specific column in a table or

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 data type of the column being operated upon Applicable data types: Numeric Applicable table types: standard tables and supertables 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` ```sql FIRST(expr) ``` Description: The first non-null value of a specific column in a table or STable Return value type:Same as the data type of the column being operated upon Applicable data types: Numeric Applicable table types: standard tables and supertables More explanation: FIRST(\) can be used to get the first non-null value of all columns; When querying a super table and multiResultFunctionStarReturnTags is set to 0 (default), FIRST(\) only returns columns of super table; When set to 1, returns columns and tags of the super table. NULL will be returned if all the values of the specified column are all NULL A result will NOT be returned if all the columns in the result set are all NULL For a table with composite primary key, the data with the smallest primary key value is returned. ```sql INTERP(expr [, ignorenullvalues]) ignorenullvalues: { 0 | 1 } ``` Description: The value that matches the specified timestamp range is returned, if existing; or an interpolation value is returned. The value of `ignorenullvalues` can be 0 or 1, 1 means null values are ignored. The default value of this parameter is 0. Return value type: Same as the column being operated upon Applicable data types: Numeric Applicable table types: standard tables and supertables 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 and a `where` clause can be used to filter the original data. If no `where` condition is specified then all original data is the input. `INTERP` must be used along with `RANGE`, `EVERY`, `FILL` keywords. The output time range of `INTERP` is specified by `RANGE(timestamp1,timestamp2)` parameter, with timestamp1 &lt;= timestamp2. timestamp1 is the starting point of the output time range. timestamp2 is the ending point of the output time range. The number of rows in the result set of `INTERP` is determined by the parameter `EVERY(timeunit)`. Starting from timestamp1, one interpolation is performed for every time interval specified `timeunit` parameter. The parameter `time_unit` must be an integer, with no quotes, with a time unit of: a(millisecond)), s(second), m(minute), h(hour), d(day), or w(week). For example, `EVERY(500a)` will interpolate every 500 milliseconds. Interpolation is performed based on `FILL` parameter. For more information about FILL clause, see . When only one timestamp value is specified in `RANGE` clause, `INTERP` is used to generate interpolation at this point in time. In this case, `EVERY` clause can be omitted. For example, SELECT INTERP(col) FROM tb RANGE('2023-01-01 00:00:00') FILL(linear). `INTERP` can be applied to supertable by interpolating primary key sorted data of all its childtables. It can also be used with `partition by tbname` when applied to supertable to generate interpolation on each single timeline. Pseudocolumn `_irowts` can be used along with `INTERP` to return the timestamps associated with interpolation points(support after version 3.0.2.0). Pseudocolumn `_isfilled` can be used along with `INTERP` to indicate whether the results are original records or data points generated by interpolation algorithm(support after version

For a table with composite primary key, onley the data with the smallest primary key value is used to generate interpolation. Example We use the smart meters example used in this documentation to illustrate how to use the INTERP function. We want to downsample every 1 hour and use a linear fill for missing values. Note the order in which the "partition by" clause and the "range", "every" and "fill" parameters are used. ```sql SELECT _irowts,INTERP(current) FROM test.meters PARTITION BY TBNAME RANGE('2017-07-22 00:00:00','2017-07-24 12:25:00') EVERY(1h) FILL(LINEAR) ``` ```sql LAST(expr) ``` Description: The last non-NULL value of a specific column in a table or STable Return value type:Same as the data type of the column being operated upon Applicable data types: Numeric Applicable table types: standard tables and supertables More explanation: LAST(\) can be used to get the last non-NULL value of all columns; When querying a super table and multiResultFunctionStarReturnTags is set to 0 (default), LAST(\) only returns columns of super table; When set to 1, returns columns and tags of the super table. 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. For a table with composite primary key, the data with the largest primary key value is returned. ```sql LAST_ROW(expr) ``` Description: The last row of a table or STable Return value type:Same as the data type of the column being operated upon Applicable data types: Numeric Applicable table types: standard tables and supertables More explanations: LASTROW(\*) can be used to get the last value of all columns; When querying a super table and multiResultFunctionStarReturnTags is set to 0 (default), LASTROW(\*) only returns columns of super table; When set to 1, returns columns and tags of the super table. 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`. Like `LAST`, the data with the largest primary key value is returned for a table with composite primary key. ```sql MAX(expr) ``` 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 Applicable data types: Numeric Applicable table types: standard tables and supertables ```sql MIN(expr) ``` 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 Applicable data types: Numeric Applicable table types: standard tables and supertables ```sql MODE(expr) ``` Description:The value which has the highest frequency of occurrence. One random value is returned if there are multiple values which have highest frequency of occurrence. Return value type: Same as the input data Applicable data types: Numeric Applicable table types: standard tables and supertables ```sql SAMPLE(expr, k) ``` Description: k sampling values of a specific column. The applicable range of k is

Return value type: Same as the column being operated Applicable data types: Any data type Applicable nested query: Inner query and Outer query Applicable table types: standard tables and supertables ```sql TAIL(expr, k, offset_val) ``` Description: The next k rows are returned after skipping the last `offsetval` rows, NULL values are not ignored. `offsetval` is optional parameter. When it's not specified, the last k rows are returned. When `offsetval` is used, the effect is same as `order by ts desc LIMIT k OFFSET offsetval`. Parameter value range: k: [1,100] offset_val: [0,100] Return value type:Same as the data type of 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 ```sql TOP(expr, k) ``` 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 data type of the column being operated upon Applicable data types: Numeric Applicable table types: standard tables and supertables 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` ```sql UNIQUE(expr) ``` Description: The values that occur the first time in the specified column. The effect is similar to `distinct` keyword. For a table with composite primary key, only the data with the smallest primary key value is returned. Return value type:Same as the data type of the column being operated upon Applicable column types: Any data types Applicable table types: table, STable 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. ```sql CSUM(expr) ``` Description: The cumulative sum of each row for a specific column, NULL value will be discard. Return value type: Long integer for integers; Double for floating points. uint64_t for unsigned integers Applicable data types: Numeric 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: 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. ```sql DERIVATIVE(expr, timeinerval, ignorenegative) ignore_negative: { 0 | 1 } ``` Description: The derivative of a specific column. The time rage can be specified by parameter `timeinterval`, the minimum allowed time range is 1 second (1s); the value of `ignorenegative` can be 0 or 1, 1 means negative values are ignored. For tables with composite primary key, the data with the smallest primary key value is used to calculate the derivative. Return value type: DOUBLE Applicable data types: Numeric Applicable table types: standard tables and supertables More explanation: It can be used together with a selected column. For example: select \_rowts, DERIVATIVE() from. ```sql DIFF(expr [, ignore_negative]) ignore_negative: { 0 | 1 } ``` 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. For tables with composite primary key, the data with the smallest primary key value is used to calculate the

Return value type:Same as the data type of the column being operated upon Applicable data types: Numeric Applicable table types: standard tables and supertables 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. ```sql IRATE(expr) ``` 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. For tables with composite primary key, the data with the smallest primary key value is used to calculate the rate. Return value type: DOUBLE Applicable data types: Numeric Applicable table types: standard tables and supertables ```sql MAVG(expr, k) ``` 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 Applicable data types: Numeric 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: 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. ```sql STATECOUNT(expr, oper, val) ``` 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 equal to), `'GE'` (greater than or equal to), `'NE'` (not equal to), `'EQ'` (equal to), the value is case insensitive, the value must be in quotes. val: Numeric types Return value type: Integer Applicable data types: Numeric Applicable nested query: Outer query only Applicable table types: standard tables and supertables More explanations: Can't be used with window operation, like interval/statewindow/sessionwindow ```sql STATEDURATION(expr, oper, val, unit) ``` 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 equal to), `'GE'` (greater than or equal to), `'NE'` (not equal to), `'EQ'` (equal to), the value is case insensitive, the value must be in quotes. 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 Applicable data types: Numeric Applicable nested query: Outer query only Applicable table types: standard tables and supertables More explanations: Can't be used with window operation, like interval/statewindow/sessionwindow ```sql TWA(expr) ``` Description: Time weighted average on a specific column within a time

For tables with composite primary key, the data with the smallest primary key value is used to calculate the average. Return value type: DOUBLE Applicable data types: Numeric Applicable table types: standard tables and supertables ```sql SELECT DATABASE(); ``` 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()`. ```sql SELECT CLIENT_VERSION(); ``` Description: The client version. ```sql SELECT SERVER_VERSION(); ``` Description: The server version. ```sql SELECT SERVER_STATUS(); ``` Description: The server status. ```sql SELECT CURRENT_USER(); ``` Description: get current user. Geometry input functions create geometry data from WTK. ```sql ST_GeomFromText(VARCHAR WKT expr) ``` Description: Return a specified GEOMETRY value from Well-Known Text representation (WKT). Return value type: GEOMETRY Applicable data types: VARCHAR Applicable table types: standard tables and supertables Explanations The input can be one of WTK string, like POINT, LINESTRING, POLYGON, MULTIPOINT, MULTILINESTRING, MULTIPOLYGON, GEOMETRYCOLLECTION. The output is a GEOMETRY data type, internal defined as binary string. Geometry output functions convert geometry data into WTK. ```sql ST_AsText(GEOMETRY geom) ``` Description: Return a specified Well-Known Text representation (WKT) value from GEOMETRY data. Return value type: VARCHAR Applicable data types: GEOMETRY Applicable table types: standard tables and supertables Explanations The output can be one of WTK string, like POINT, LINESTRING, POLYGON, MULTIPOINT, MULTILINESTRING, MULTIPOLYGON, GEOMETRYCOLLECTION. Geometry relationships functions determine spatial relationships between geometries. ```sql ST_Intersects(GEOMETRY geomA, GEOMETRY geomB) ``` Description: Compares two geometries and returns true if they intersect. Return value type: BOOL Applicable data types: GEOMETRY, GEOMETRY Applicable table types: standard tables and supertables Explanations Geometries intersect if they have any point in common. ```sql ST_Equals(GEOMETRY geomA, GEOMETRY geomB) ``` Description: Returns TRUE if the given geometries are "spatially equal". Return value type: BOOL Applicable data types: GEOMETRY, GEOMETRY Applicable table types: standard tables and supertables Explanations 'Spatially equal' means STContains(A,B) = true and STContains(B,A) = true, and the ordering of points can be different but represent the same geometry structure. ```sql ST_Touches(GEOMETRY geomA, GEOMETRY geomB) ``` Description: Returns TRUE if A and B intersect, but their interiors do not intersect. Return value type: BOOL Applicable data types: GEOMETRY, GEOMETRY Applicable table types: standard tables and supertables Explanations A and B have at least one point in common, and the common points lie in at least one boundary. For Point/Point inputs the relationship is always FALSE, since points do not have a boundary. ```sql ST_Covers(GEOMETRY geomA, GEOMETRY geomB) ``` Description: Returns TRUE if every point in Geometry B lies inside (intersects the interior or boundary of) Geometry A. Return value type: BOOL Applicable data types: GEOMETRY, GEOMETRY Applicable table types: standard tables and supertables Explanations A covers B means no point of B lies outside (in the exterior of) A. ```sql ST_Contains(GEOMETRY geomA, GEOMETRY geomB) ``` Description: Returns TRUE if geometry A contains geometry B. Return value type: BOOL Applicable data types: GEOMETRY, GEOMETRY Applicable table types: standard tables and supertables Explanations A contains B if and only if all points of B lie inside (i.e. in the interior or boundary of) A (or equivalently, no points of B lie in the exterior of A), and the interiors of A and B have at least one point in common. ```sql ST_ContainsProperly(GEOMETRY geomA, GEOMETRY geomB) ``` Description: Returns TRUE if every point of B lies inside A. Return value type: BOOL Applicable data types: GEOMETRY, GEOMETRY Applicable table types: standard tables and supertables Explanations There is no point of B that lies on the boundary of A or in the exterior of A.