SQL中Having與Where的區別

含義

where

在分組之前就進行選擇操作

having

having在分組操作之後才進行選擇操作，所以having可以選擇聚合函數
eg：

// 下面這句是可以的
SELECT COUNT(*) FROM 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；

//必須要改為having
SELECT COUNT(*) FROM A
WHERE A.a1>0
GROUP BY a2
HAVING COUNT(*)>1