select u.university ,q.difficult_level, count(qp.id)/count(distinct qp.device_id) from user_profile u inner join question_practice_detail qp on qp.device_id = u.device_id inner join question_detail q on q.question_id = qp.question_id group by u.university,q.difficult_level 三表连接,感觉题目理解比较重要。practice表...