题解 | #统计每个学校的答过题的用户的平均答题数#

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