**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.
**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.
**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.
**Applicable table types**: standard tables and supertables
**Explanations**：
- _P_ is in range [0,100], when _P_ is 0, the result is same as using function MIN; when _P_ is 100, the result is same as function MAX.
- _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` 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.
### AVG
```sql
SELECTAVG(field_name)FROMtb_name[WHEREclause];
AVG(expr)
```
**Description**: The average value of the specified fields.
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@@ -633,7 +634,7 @@ SELECT AVG(field_name) FROM tb_name [WHERE clause];
**Description**：`elapsed` function can be used to calculate the continuous time length in which there is valid data. If it's used with `INTERVAL` clause, the returned result is the calcualted time length within each time window. If it's used without `INTERVAL` caluse, the returned result is the calculated time length within the specified time range. Please be noted that the return value of `elapsed` is the number of `time_unit` in the calculated time length.
**Applicable tables**: table, STable, outter in nested query
**Explanations**：
-`field_name` parameter can only be the first column of a table, i.e. timestamp primary key.
-`ts_primary_key` parameter can only be the first column of a table, i.e. timestamp primary key.
- The minimum value of `time_unit` is the time precision of the database. If `time_unit` is not specified, the time precision of the database is used as the default time unit. Time unit specified by `time_unit` can be:
- It can be used with `INTERVAL` to get the time valid time length of each time window. Please be noted that the return value is same as the time window for all time windows except for the first and the last time window.
SELECT LEASTSQUARES(field_name, start_val, step_val) FROM tb_name [WHERE clause];
LEASTSQUARES(expr, start_val, step_val)
```
**Description**: The linear regression function of the specified column and the timestamp column (primary key), `start_val` is the initial value and `step_val` is the step value.
**Description**：Returns count of data points in user-specified ranges.
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@@ -786,7 +787,7 @@ SELECT HISTOGRAM(field_name，bin_type, bin_description, normalized) FROM tb_nam
### PERCENTILE
```sql
SELECT PERCENTILE(field_name, P) FROM { tb_name } [WHERE clause];
PERCENTILE(expr, p)
```
**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.
**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.
**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.
## Selection Functions
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@@ -807,7 +808,7 @@ Selection functions return one or more results depending. You can specify the ti
**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.
SELECT MODE(field_name) FROM tb_name [WHERE clause];
MODE(expr)
```
**Description**:The value which has the highest frequency of occurrence. NULL is returned if there are multiple values which have highest frequency of occurrence.
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@@ -955,7 +956,7 @@ SELECT MODE(field_name) FROM tb_name [WHERE clause];
**Description**: _k_ sampling values of a specific column. The applicable range of _k_ is [1,1000].
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@@ -977,7 +978,7 @@ This function cannot be used in expression calculation.
### TAIL
```sql
SELECT TAIL(field_name, k, offset_val) FROM {tb_name | stb_name} [WHERE clause];
TAIL(expr, k, offset_val)
```
**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`.
**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.
SELECT UNIQUE(field_name) FROM {tb_name | stb_name} [WHERE clause];
UNIQUE(expr)
```
**Description**: The values that occur the first time in the specified column. The effect is similar to `distinct` keyword, but it can also be used to match tags or timestamp. The first occurrence of a timestamp or tag is used.
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@@ -1033,7 +1034,7 @@ TDengine includes extensions to standard SQL that are intended specifically for
### CSUM
```sql
SELECT CSUM(field_name) FROM { tb_name | stb_name } [WHERE clause]
CSUM(expr)
```
**Description**: The cumulative sum of each row for a specific column. The number of output rows is same as that of the input rows.
SELECT DERIVATIVE(field_name, time_interval, ignore_negative) FROM tb_name [WHERE clause];
DERIVATIVE(expr, time_inerval, ignore_negative)
ignore_negative: {
0
| 1
}
```
**Description**: The derivative of a specific column. The time rage can be specified by parameter `time_interval`, the minimum allowed time range is 1 second (1s); the value of `ignore_negative` can be 0 or 1, 1 means negative values are ignored.
SELECT {DIFF(field_name, ignore_negative) | DIFF(field_name)} FROM tb_name [WHERE clause];
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.
SELECT IRATE(field_name) FROM tb_name WHERE clause;
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.
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@@ -1110,7 +1121,7 @@ SELECT IRATE(field_name) FROM tb_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 of _k_ is [1,1000].
**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.
**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.
**功能说明**：返回跳过最后 offset_val 个，然后取连续 k 个记录，不忽略 NULL 值。offset_val 可以不输入。此时返回最后的 k 个记录。当有 offset_val 输入的情况下，该函数功能等效于 `order by ts desc LIMIT k OFFSET offset_val`。
