题解 | 统计每个用户的平均刷题数

SELECT
    U.university,
    QD.difficult_level,
    ROUND(COUNT(Q.question_id)/COUNT(DISTINCT(Q.device_id)),4) AS avg_answer_cnt
FROM
    user_profile U,
    question_practice_detail Q,
    question_detail QD
WHERE 
    U.university= '山东大学' AND
    U.device_id = Q.device_id AND
    Q.question_id = QD.question_id
GROUP BY
    QD.difficult_level