# 计算每个学校用户不同难度下的用户平均答题题目数情况 select university, difficult_level, count(q.question_id)/count(distinct u.device_id) from question_practice_detail q join user_profile u on u.device_id = q.device_id join question_detail qd on q.question_id = qd.question_id group by university, difficult_level ;