题解 | #统计每个用户的平均刷题数#
统计每个用户的平均刷题数
http://www.nowcoder.com/practice/f4714f7529404679b7f8909c96299ac4
SELECT university,difficult_level,
COUNT(question_detail.question_id)/COUNT(DISTINCT user_profile.device_id)
AS avg_answer_cnt
FROM user_profile
INNER JOIN question_practice_detail ON user_profile.device_id=question_practice_detail.device_id
INNER JOIN question_detail ON question_detail.question_id=question_practice_detail.question_id
where university='山东大学'
GROUP BY difficult_level
两个条件分开列，要主要在多个表中出现的字段要定义是哪一个表格
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