SQL中Having与Where的区别

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