题解 | #统计每个学校各难度的用户平均刷题数#
统计每个学校各难度的用户平均刷题数
https://www.nowcoder.com/practice/5400df085a034f88b2e17941ab338ee8
select university , difficult_level , count(question_id) / count(distinct r.device_id) as avg_answer_cnt from ( select question_practice_detail.question_id , question_detail.difficult_level , device_id from question_practice_detail join question_detail on question_detail.question_id = question_practice_detail.question_id ) r join user_profile z on r.device_id = z.device_id group by university,difficult_level
采用字句优化join查询效率