题解 | 统计每个用户的平均刷题数
统计每个用户的平均刷题数
https://www.nowcoder.com/practice/f4714f7529404679b7f8909c96299ac4
select a.university,c.difficult_level,count(b.id)/count(distinct b.device_id) avg_answer_cnt from user_profile a,question_practice_detail b,question_detail c where a.university='山东大学'and a.device_id=b.device_id and b.question_id=c.question_id group by university,difficult_level