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