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:
// 下面这句是可以的 select count(*) from a where a.a1>0 group by a2;
//但是下面就不行 select count(*) from a where a.a1>0 and count(*)>1 group by a2;
//必须要改为having select count(*) from a where a.a1>0 group by a2 having count(*)>1
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