含義
where
在分組之前就進行選擇操作
having
having在分組操作之后才進行選擇操作，所以having可以選擇聚合函數
eg：
// 下面這句是可以的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；
//必須要改為havingSELECT COUNT(*) FROM AWHERE A.a1>0GROUP BY a2HAVING COUNT(*)>1
