COUNT(*)是否总是返回结果?

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如果我运行查询，例如:

If I run a query such as:

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

即使查询与任何记录都不匹配，它也会始终返回结果吗?还是我需要验证并确保结果返回一行?

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且返回零. 除非您添加GROUP BY，否则在这种情况下将没有结果，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，然后没有任何行. 只有COUNT个返回没有结果的数字

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: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.我猜是按照ANSI标准

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