SQL 函数调用被“HAVING"忽略条款 [英] SQL Function calls ignored with "HAVING" clause

问题描述

对于下面的查询，MAX 函数似乎被完全忽略了.MAX函数对查询结果没有影响

For the below query, the MAX function seems to be completely ignored. The MAX function has no effect on the result of the query

SELECT alarm_severity_id
FROM alarm_notification
WHERE alarm_life_cycle_id = 25
having MAX(event_timestamp);

推荐答案

这是您的查询:

SELECT alarm_severity_id
FROM alarm_notification
WHERE alarm_life_cycle_id = 25
HAVING MAX(event_timestamp);

您的 HAVING 子句说:取时间戳的最大值.如果最大值不等于零或 NULL，则一切正常.这完全等效:

Your HAVING clause is saying: take the maximum of the timestamp. If the maximum is not equal to zero or NULL, then everything is fine. This is exactly equivalent:

HAVING MAX(event_timestamp) <> 0

我怀疑你想要:

SELECT alarm_severity_id
FROM alarm_notification
WHERE alarm_life_cycle_id = 25
ORDER BY event_timestamp DESC
LIMIT 1;

要优化性能，请在 alarm_life_cycle_id 和 event_timestamp 上创建索引:

To optimize performance, create an index on alarm_life_cycle_id and event_timestamp:

create index alarm_notification_alci_et on alarm_notification(alarm_life_cycle_id, event_timestamp)

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