题解 | #统计每个学校各难度的用户平均刷题数#
统计每个学校各难度的用户平均刷题数
http://www.nowcoder.com/practice/5400df085a034f88b2e17941ab338ee8
1.两张小表先关联
2.再和 user_profile表关联
3.分组 group by university,difficult_level
4.count(question_id)/count(distinct u.device_id) 总的答题数量除答题人数
select university,difficult_level,count(question_id)/count(distinct u.device_id) from user_profile u,(select x.*,y.difficult_level from question_practice_detail x,question_detail y where x.question_id = y.question_id) as m where u.device_id = m.device_id group by university,difficult_level
