SQL中Having与Where的区别

// 下面这句是可以的SELECT COUNT(*) FROM AWHERE A.a1>0GROUP BY a2；

//但是下面就不行SELECT COUNT(*) FROM AWHERE A.a1>0 and count(*)>1GROUP BY a2；

//必须要改为havingSELECT COUNT(*) FROM AWHERE A.a1>0GROUP BY a2HAVING COUNT(*)>1