题解 | #统计每个用户的平均刷题数#

SELECT
    university,difficult_level,
    ROUND(COUNT(qpd.question_id)/COUNT(DISTINCT(qpd.device_id)),4) AS avg_answer_cnt
	##注意精确到小数点后4位
FROM
    question_practice_detail AS qpd
INNER JOIN
    user_profile AS u
ON
    qpd.device_id=u.device_id
INNER JOIN
    question_detail AS qd
ON
    qpd.question_id=qd.question_id
WHERE
    u.university='山东大学'
	#比上一题多一个where条件
GROUP BY
    difficult_level;