# 查询参与了答题的山东大学在不同难度下的答题总数和答题用户数
select university, difficult_level, round(count(*)/count(distinct u.device_id),4) avg_answer_cnt
from user_profile u                     # 多表连接
join question_practice_detail q
on u.device_id=q.device_id
join question_detail d
on q.question_id=d.question_id
where university='山东大学'             # 筛选条件，筛选出山东大学
group by university, difficult_level;