题解 | #统计每个用户的平均刷题数#

SELECT us.university,
q1.difficult_level,
count(q.question_id)/count(distinct q.device_id) as avg_answer_cnt
FROM user_profile as us
JOIN question_practice_detail as q
ON us.device_id=q.device_id
JOIN question_detail as q1
ON q1.question_id=q.question_id
GROUP BY us.university,q1.difficult_level
HAVING us.university='山东大学'

这里有个小卡点就是GTOUP BY那里不要少写个 us.university