题解 | 统计每个学校各难度的用户平均刷题数
select university , difficult_level , round(count(qpd.question_id)/count(distinct qpd.device_id),4) avg_answer_cnt from question_practice_detail qpd left join user_profile up on qpd.device_id=up.device_id left join question_detail qd on qpd.question_id=qd.question_id group by university, difficult_level
