@@ -8,11 +8,11 @@ Window related clauses are used to divide the data set to be queried into subset
## Time Window
`INTERVAL` clause is used to generate time windows of same time interval, `SLIDING` is used to specify the time step for which the time window moves forward. The query is performed on one time window each time, and the time window moves forward with time. When defining continuous query both the size of time window and the step of forward sliding time need to be specified. As shown in the figure blow, [t0s, t0e] ，[t1s , t1e]， [t2s, t2e] are respectively the time range of three time windows on which continuous queries are executed. The time step for which time window moves forward is marked by `sliding time`. Query, filter and aggregate operations are executed on each time window respectively. When the time step specified by `SLIDING` is same as the time interval specified by `INTERVAL`, the sliding time window is actually a flip time window.
`INTERVAL` clause is used to generate time windows of the same time interval, `SLIDING` is used to specify the time step for which the time window moves forward. The query is performed on one time window each time, and the time window moves forward with time. When defining continuous query both the size of time window and the step of forward sliding time need to be specified. As shown in the figure blow, [t0s, t0e] ，[t1s , t1e]， [t2s, t2e] are respectively the time ranges of three time windows on which continuous queries are executed. The time step for which time window moves forward is marked by `sliding time`. Query, filter and aggregate operations are executed on each time window respectively. When the time step specified by `SLIDING` is same as the time interval specified by `INTERVAL`, the sliding time window is actually a flip time window.
`INTERVAL` and `SLIDING` should be used with aggregate functions and selection functions. Below SQL statement is illegal because no aggregate or selection function is used with `INTERVAL`.
`INTERVAL` and `SLIDING` should be used with aggregate functions and select functions. Below SQL statement is illegal because no aggregate or selection function is used with `INTERVAL`.
```
SELECT * FROM temp_tb_1 INTERVAL(1m);
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@@ -24,11 +24,11 @@ The time step specified by `SLIDING` can't exceed the time interval specified by
SELECT COUNT(*) FROM temp_tb_1 INTERVAL(1m) SLIDING(2m);
```
When the time length specified by `SLIDING` is same as that specified by `INTERVAL`, sliding window is actually flip window. The minimum time range specified by `INTERVAL` is 10 milliseconds (10a) prior to version 2.1.5.0. From version 2.1.5.0, the minimum time range by `INTERVAL` can be 1 microsecond (1u). However, if the DB precision is millisecond, the minimum time range is 1 millisecond (1a). Please be noted that the `timezone` parameter should be configured to same value in the `taos.cfg` configuration file on client side and server side.
When the time length specified by `SLIDING` is the same as that specified by `INTERVAL`, the sliding window is actually a flip window. The minimum time range specified by `INTERVAL` is 10 milliseconds (10a) prior to version 2.1.5.0. From version 2.1.5.0, the minimum time range by `INTERVAL` can be 1 microsecond (1u). However, if the DB precision is millisecond, the minimum time range is 1 millisecond (1a). Please note that the `timezone` parameter should be configured to be the same value in the `taos.cfg` configuration file on client side and server side.
## Status Window
In case of using integer, bool, or string to represent the device status at a moment, the continuous rows with same status belong to same status window. Once the status changes, the status window closes. As shown in the following figure，there are two status windows according to status, [2019-04-28 14:22:07，2019-04-28 14:22:10] and [2019-04-28 14:22:11，2019-04-28 14:22:12]. Status window is not applicable to STable for now.
In case of using integer, bool, or string to represent the device status at a moment, the continuous rows with same status belong to same status window. Once the status changes, the status window closes. As shown in the following figure, there are two status windows according to status, [2019-04-28 14:22:07，2019-04-28 14:22:10] and [2019-04-28 14:22:11，2019-04-28 14:22:12]. Status window is not applicable to STable for now.
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@@ -44,7 +44,7 @@ SELECT COUNT(*), FIRST(ts), status FROM temp_tb_1 STATE_WINDOW(status);
The primary key, i.e. timestamp, is used to determine which session window the row belongs to. If the time interval between two adjacent rows is within the time range specified by `tol_val`, they belong to same session window; otherwise they belong to two different time windows. As shown in the figure below, if the limit of time interval for session window is specified as 12 seconds, then the 6 rows in the figure constitutes 2 time windows, [2019-04-28 14:22:10，2019-04-28 14:22:30] and [2019-04-28 14:23:10，2019-04-28 14:23:30], because the time difference between 2019-04-28 14:22:30 and 2019-04-28 14:23:10 is 40 seconds, which exceeds the time interval limit of 12 seconds.
The primary key, i.e. timestamp, is used to determine which session window the row belongs to. If the time interval between two adjacent rows is within the time range specified by `tol_val`, they belong to the same session window; otherwise they belong to two different time windows. As shown in the figure below, if the limit of time interval for the session window is specified as 12 seconds, then the 6 rows in the figure constitutes 2 time windows, [2019-04-28 14:22:10，2019-04-28 14:22:30] and [2019-04-28 14:23:10，2019-04-28 14:23:30], because the time difference between 2019-04-28 14:22:30 and 2019-04-28 14:23:10 is 40 seconds, which exceeds the time interval limit of 12 seconds.
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@@ -54,7 +54,7 @@ If the time interval between two continuous rows are within the time interval sp
### Syntax
The full syntax of aggregate by window is as following:
The full syntax of aggregate by window is as follows:
```sql
SELECTfunction_listFROMtb_name
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@@ -73,11 +73,11 @@ SELECT function_list FROM stb_name
### Restrictions
- Aggregate functions and selection functions can be used in `function_list`, with each function having only one output, for example COUNT, AVG, SUM, STDDEV, LEASTSQUARES, PERCENTILE, MIN, MAX, FIRST, LAST. Functions having multiple output can't be used, for example DIFF or arithmetic operations.
- Aggregate functions and select functions can be used in `function_list`, with each function having only one output, for example COUNT, AVG, SUM, STDDEV, LEASTSQUARES, PERCENTILE, MIN, MAX, FIRST, LAST. Functions having multiple output can't be used, for example DIFF or arithmetic operations.
-`LAST_ROW` can't be used together with window aggregate.
- Scalar functions, like CEIL/FLOOR, can't be used with window aggregate.
-`WHERE` clause can be used to specify the starting and ending time and other filter conditions
-`FILL` clause is used to specify how to fill when there is data missing in any window, including:\
-`FILL` clause is used to specify how to fill when there is data missing in any window, including:
1. NONE: No fill (the default fill mode)
2. VALUE：Fill with a fixed value, which should be specified together, for example `FILL(VALUE, 1.23)`
3. PREV：Fill with the previous non-NULL value, `FILL(PREV)`
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@@ -88,21 +88,22 @@ SELECT function_list FROM stb_name
:::info
1. Huge volume of interpolation output may be returned using `FILL`, so it's recommended to specify the time range when using `FILL`. The maximum interpolation values that can be returned in single query is 10,000,000.
2. The result set is in the ascending order of timestamp in aggregate by time window aggregate.
2. The result set is in ascending order of timestamp in aggregate by time window aggregate.
3. If aggregate by window is used on STable, the aggregate function is performed on all the rows matching the filter conditions. If `GROUP BY` is not used in the query, the result set will be returned in ascending order of timestamp; otherwise the result set is not exactly in the order of ascending timestamp in each group.
:::
:::
Aggregate by time window is also used in continuous query, please refer to [Continuous Query](/develop/continuous-query).
## Examples
The table of intelligent meters can be created like below SQL statement：
The table of intelligent meters can be created by the SQL statement below：
The average current, maximum current and median of current in every 10 minutes of the past 24 hours can be calculated using below SQL statement, with missing value filled with the previous non-NULL value.
The average current, maximum current and median of current in every 10 minutes for the past 24 hours can be calculated using the below SQL statement, with missing values filled with the previous non-NULL values.
```
SELECT AVG(current), MAX(current), APERCENTILE(current, 50) FROM meters
