题解 | 统计每个用户的平均刷题数
select u.university, qd.difficult_level, round(count(qpd.question_id)/count(distinct(qpd.device_id)),4) from user_profile u join question_practice_detail qpd on u.device_id = qpd.device_id join question_detail qd on qpd.question_id = qd.question_id where university like "山东大学" group by university,difficult_level