题解 | #统计每个用户的平均刷题数#

SELECT 
    t1.university,     -- t1 user_pro
    t3.difficult_level,-- t3 ques_detail
    COUNT(t2.question_id) / COUNT(DISTINCT t2.device_id) avg_answer_cnt -- 总答题数 / 总用户数
FROM user_profile t1
JOIN question_practice_detail t2
    ON t1.device_id = t2.device_id
JOIN question_detail t3
    ON t2.question_id = t3.question_id
WHERE t1.university = "山东大学"
GROUP BY t3.difficult_level;