题解 | #统计每个学校的答过题的用户的平均答题数#

SELECT up.university,
       COUNT(qpd.device_id)/COUNT(DISTINCT qpd.device_id)
            AS avg_answer_cnt 
FROM user_profile AS up
INNER JOIN question_practice_detail AS qpd
ON up.device_id = qpd.device_id
GROUP BY up.university