where不能筛选通过函数计算出来的列。
统计消费过100元以上的用户：
错误1
select userid, ordernum, sum(price) from orders group by (userid) where sum(price) > 100;
错误2(设置别名也不行)
select userid, ordernum, sum(price) as spent from orders group by (userid) where spent > 100;
正确：
select userid, ordernum, sum(price) from orders group by (userid) having sum(price) > 100;
