题解 | 统计每个用户的平均刷题数

select u.university, qd.difficult_level, count(q.device_id)/count(distinct q.device_id) avg_answer_cnt
from user_profile u
inner join question_practice_detail q
on u.device_id = q.device_id
inner join question_detail qd
on q.question_id = qd.question_id
where u.university = '山东大学'
group by u.university, qd.difficult_level