COUNT(*) 是否总是返回结果?

2021-11-20 00:00:00 sql aggregate mysql

如果我运行一个查询，例如:

If I run a query such as:

SELECT COUNT(*) as num FROM table WHERE x = 'y'

它是否总是返回结果，即使查询不匹配任何记录?或者我是否需要验证并确保返回一行作为结果?

Will it always return a result, even when the query doesn't match any record? Or do I need to validate and make sure a row is returned as the result?

推荐答案

是的，因为它是一个聚合并返回零.除非您添加 GROUP BY 在这种情况下没有结果，因为没有组...

Yes, because it's an aggregate and returns zero. Unless you add GROUP BY in which case no result because there is no group...

MAX/SUM 等将返回 NULL，除非您添加 GROUP BY 然后没有行.只有 COUNT 返回一个没有结果的数字

MAX/SUM etc would return NULL unless you add GROUP BY then no rows. Only COUNT returns a number for no results

编辑，有点晚了:SUM 会像 MAX 一样返回 NULL

Edit, a bit late: SUM would return NULL like MAX

编辑，2013 年 5 月:这适用于所有主要 RDBMS.我想按照 ANSI 标准

Edit, May 2013: this applies to all main RDBMS. I guess as per the ANSI standard

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