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| |
| ## 聚合函数 |
| |
| 聚合函数是多对一函数。它们对一组值进行聚合计算，得到单个聚合结果。 |
| |
| 除了 `COUNT()`, `COUNT_IF()`之外，其他所有聚合函数都忽略空值，并在没有输入行或所有值为空时返回空值。 例如，`SUM()` 返回 null 而不是零，而 `AVG()` 在计数中不包括 null 值。 |
| |
| IoTDB 支持的聚合函数如下： |
| |
| | 函数名 | 功能描述 | 允许的输入类型 | 必要的属性参数 | 输出类型 | |
| | ----------- |-----------------------------------------------| ------------------------ |----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------| |
| | SUM | 求和。 | INT32 INT64 FLOAT DOUBLE | 无 | DOUBLE | |
| | COUNT | 计算数据点数。 | 所有类型 | 无 | INT64 | |
| | AVG | 求平均值。 | INT32 INT64 FLOAT DOUBLE | 无 | DOUBLE | |
| | EXTREME | 求具有最大绝对值的值。如果正值和负值的最大绝对值相等，则返回正值。 | INT32 INT64 FLOAT DOUBLE | 无 | 与输入类型一致 | |
| | MAX_VALUE | 求最大值。 | INT32 INT64 FLOAT DOUBLE | 无 | 与输入类型一致 | |
| | MIN_VALUE | 求最小值。 | INT32 INT64 FLOAT DOUBLE | 无 | 与输入类型一致 | |
| | FIRST_VALUE | 求时间戳最小的值。 | 所有类型 | 无 | 与输入类型一致 | |
| | LAST_VALUE | 求时间戳最大的值。 | 所有类型 | 无 | 与输入类型一致 | |
| | MAX_TIME | 求最大时间戳。 | 所有类型 | 无 | Timestamp | |
| | MIN_TIME | 求最小时间戳。 | 所有类型 | 无 | Timestamp | |
| | COUNT_IF | 求数据点连续满足某一给定条件，且满足条件的数据点个数（用keep表示）满足指定阈值的次数。 | BOOLEAN | `[keep >=/>/=/!=/</<=]threshold`：被指定的阈值或阈值条件，若只使用`threshold`则等价于`keep >= threshold`,`threshold`类型为`INT64`<br/> `ignoreNull`：可选，默认为`true`；为`true`表示忽略null值，即如果中间出现null值，直接忽略，不会打断连续性；为`false`表示不忽略null值，即如果中间出现null值，会打断连续性 | INT64 | |
| |
| ### COUNT_IF |
| |
| #### 语法 |
| ```sql |
| count_if(predicate, [keep >=/>/=/!=/</<=]threshold[, 'ignoreNull'='true/false']) |
| ``` |
| predicate: 返回类型为`BOOLEAN`的合法表达式 |
| |
| threshold 及 ignoreNull 用法见上表 |
| |
| >注意: count_if 当前暂不支持与 group by time 的 SlidingWindow 一起使用 |
| |
| #### 使用示例 |
| |
| ##### 原始数据 |
| |
| ``` |
| +-----------------------------+-------------+-------------+ |
| | Time|root.db.d1.s1|root.db.d1.s2| |
| +-----------------------------+-------------+-------------+ |
| |1970-01-01T08:00:00.001+08:00| 0| 0| |
| |1970-01-01T08:00:00.002+08:00| null| 0| |
| |1970-01-01T08:00:00.003+08:00| 0| 0| |
| |1970-01-01T08:00:00.004+08:00| 0| 0| |
| |1970-01-01T08:00:00.005+08:00| 1| 0| |
| |1970-01-01T08:00:00.006+08:00| 1| 0| |
| |1970-01-01T08:00:00.007+08:00| 1| 0| |
| |1970-01-01T08:00:00.008+08:00| 0| 0| |
| |1970-01-01T08:00:00.009+08:00| 0| 0| |
| |1970-01-01T08:00:00.010+08:00| 0| 0| |
| +-----------------------------+-------------+-------------+ |
| ``` |
| |
| ##### 不使用ignoreNull参数(忽略null) |
| |
| SQL: |
| ```sql |
| select count_if(s1=0 & s2=0, 3), count_if(s1=1 & s2=0, 3) from root.db.d1 |
| ``` |
| |
| 输出: |
| ``` |
| +--------------------------------------------------+--------------------------------------------------+ |
| |count_if(root.db.d1.s1 = 0 & root.db.d1.s2 = 0, 3)|count_if(root.db.d1.s1 = 1 & root.db.d1.s2 = 0, 3)| |
| +--------------------------------------------------+--------------------------------------------------+ |
| | 2| 1| |
| +--------------------------------------------------+-------------------------------------------------- |
| ``` |
| |
| ##### 使用ignoreNull参数 |
| |
| SQL: |
| ```sql |
| select count_if(s1=0 & s2=0, 3, 'ignoreNull'='false'), count_if(s1=1 & s2=0, 3, 'ignoreNull'='false') from root.db.d1 |
| ``` |
| |
| 输出: |
| ``` |
| +------------------------------------------------------------------------+------------------------------------------------------------------------+ |
| |count_if(root.db.d1.s1 = 0 & root.db.d1.s2 = 0, 3, "ignoreNull"="false")|count_if(root.db.d1.s1 = 1 & root.db.d1.s2 = 0, 3, "ignoreNull"="false")| |
| +------------------------------------------------------------------------+------------------------------------------------------------------------+ |
| | 1| 1| |
| +------------------------------------------------------------------------+------------------------------------------------------------------------+ |
| ``` |
