题解 | #分组过滤练习题#
统计每个学校各难度的用户平均刷题数
http://www.nowcoder.com/practice/5400df085a034f88b2e17941ab338ee8
select university,difficult_level,count(up.device_id)/ count(distinct(up.device_id)) from user_profile as up,question_practice_detail as qpd,question_detail as qd where up.device_id=qpd.device_id and qd.question_id=qpd.question_id group by university,difficult_level order by university
