# 分组查询每个学校每种题目难度下的答题用户数量、答题数量并计算刷题率
select university, difficult_level, round(count(*)/count(distinct u.device_id),4)
from user_profile u
join question_practice_detail q
on u.device_id=q.device_id
join question_detail d
on q.question_id=d.question_id
group by university, difficult_level;