含义
where
在分组之前就进行选择操作
having
having在分组操作之后才进行选择操作,所以having可以选择聚合函数
eg:
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// 下面这句是可以的 SELECT COUNT (*) FROM A WHERE A.a1>0 GROUP BY a2; |
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//但是下面就不行 SELECT COUNT (*) FROM A WHERE A.a1>0 and count (*)>1 GROUP BY a2; |
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//必须要改为 having SELECT COUNT (*) FROM A WHERE A.a1>0 GROUP BY a2 HAVING COUNT (*)>1 |