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
from user_profile up,question_practice_detail qpd,question_detail qd
where up.device_id=qpd.device_id and qpd.question_id=qd.question_id
group by university,difficult_level
order by university