mysql having与where的区别(个人理解)

having跟where都能作为刷选条件

但having可以作为mysql进行逻辑运算后的条件where只能作为字段筛选条件

例如：

用having筛选

select count(Friend.userID) from User left join Friend on User.id = Friend.userID group by User.id having count(Friend.userID) > 1;

查询Uesr表中每个用户在Friend里面的好友数量超过1个

用where筛选

select count(Friend.userID) from User left join Friend on User.id = Friend.userID where count(Friend.userID) > 1 group by User.id;

但是having是聚合语句     where的话效率更快

having就用在当where判断不足的时候补全