题解 | #统计每个学校各难度的用户平均刷题数#

-- 利用group by分组排序时必须有两个条件：university, difficult_level
select university, difficult_level, count(difficult_level)/count(distinct question_practice_detail.device_id) from user_profile, question_practice_detail, question_detail where user_profile.device_id = question_practice_detail.device_id and question_practice_detail.question_id = question_detail.question_id group by university, difficult_level;