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(qpd.id) / COUNT(DISTINCT up.device_id), 4) AS avg_answer_cnt
FROM 
    user_profile up
JOIN 
    question_practice_detail qpd ON up.device_id = qpd.device_id
JOIN 
    question_detail qd ON qpd.question_id = qd.question_id
GROUP BY 
    up.university, qd.difficult_level
ORDER BY 
    up.university, qd.difficult_level;