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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()` 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 | 输出类型 | |
| | ----------- | ------------------------------------------------------------ | ------------------------ | -------------- | |
| | SUM | 求和。 | INT32 INT64 FLOAT DOUBLE | DOUBLE | |
| | COUNT | 计算数据点数。 | 所有类型 | INT | |
| | AVG | 求平均值。 | INT32 INT64 FLOAT DOUBLE | 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 | 与输入类型一致 | |
| | MAX_VALUE | 求最大值。 | INT32 INT64 FLOAT DOUBLE | 与输入类型一致 | |
| | MIN_VALUE | 求最小值。 | INT32 INT64 FLOAT DOUBLE | 与输入类型一致 | |
| | FIRST_VALUE | 求时间戳最小的值。 | 所有类型 | 与输入类型一致 | |
| | LAST_VALUE | 求时间戳最大的值。 | 所有类型 | 与输入类型一致 | |
| | MAX_TIME | 求最大时间戳。 | 所有类型 | Timestamp | |
| | MIN_TIME | 求最小时间戳。 | 所有类型 | Timestamp | |
