SQL中Having与Where的区别

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