题解 | #统计每个用户的平均刷题数#
https://www.nowcoder.com/practice/f4714f7529404679b7f8909c96299ac4
如下：
select university,difficult_level,count(qpd.question_id)/count(distinct qpd.device_id) avg_answer_cnt from user_profile as upr left join question_practice_detail as qpd on upr.device_id=qpd.device_id left join question_detail as qd on qd.question_id=qpd.question_id where university='山东大学' group by university,difficult_level
