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
使用内连接
SELECT university,difficult_level,round((count(qpd.result)/count(DISTINCT qpd.device_id)),4) as avg_answer_cnt
FROM user_profile as u
INNER JOIN question_practice_detail as qpd
ON u.device_id = qpd.device_id
INNER JOIN question_detail as qd
ON qpd.question_id = qd.question_id
WHERE university = "山东大学"
GROUP BY difficult_level
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