SQL中Having与Where的区别

含义

• where
  在分组以前就进行选择操做

• having
  having在分组操做以后才进行选择操做，因此having能够选择聚合函数

eg：

// 下面这句是能够的
SELECT COUNT(*) FROM A
WHERE A.a1>0
GROUP BY a2；
//可是下面就不行
SELECT COUNT(*) FROM A
WHERE A.a1>0 and count(*)>1
GROUP BY a2；
//必需要改成having
SELECT COUNT(*) FROM A
WHERE A.a1>0
GROUP BY a2
HAVING COUNT(*)>1