题解 | 统计每个用户的平均刷题数
select university,difficult_level,round(count(qpd.question_id))/count(distinct qpd.device_id) from user_profile as up left join question_practice_detail as qpd on up.device_id=qpd.device_id left join question_detail as pd on qpd.question_id=pd.question_id group by university,difficult_level having university='山东大学';