题解 | #统计每个学校各难度的用户平均刷题数#

SELECT up.university, 
        qd.difficult_level, 
        ROUND(COUNT(qd.question_id)/COUNT(DISTINCT up.device_id),4) AS avg_answer_cnt 
FROM user_profile up
JOIN question_practice_detail qpd
    USING(device_id)
JOIN question_detail qd
    USING(question_id)
GROUP BY up.university, qd.difficult_level