SELECT university,COUNT(question_practice_detail.question_id)/COUNT(distinct(user_profile.device_id)) FROM user_profile,question_practice_detail
WHERE user_profile.device_id=question_practice_detail.device_id
GROUP BY university

看很多人的答案,在question_practice_detail统计题量,然后除以人数,人数也在question_practice_detail里面统计,不过按道理人数不是应该在user_profile里面统计咩?
系统是distinct(user_profile.device_id)和distinct(question_practice_detail)都对。