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
