-- 平均刷题数=总刷题数/总人数，按照题目难度进行分组统计，设定条件为山东大学
select a.university,
c.difficult_level,
count(b.question_id)/count(distinct a.device_id) as avg_answer_cnt
from user_profile as a
right join question_practice_detail as b
on a.device_id=b.device_id
left join question_detail as c
on b.question_id=c.question_id
where a.university='山东大学'
group by c.difficult_level;