题解 | #统计每个学校各难度的用户平均刷题数#
统计每个学校各难度的用户平均刷题数
http://www.nowcoder.com/practice/5400df085a034f88b2e17941ab338ee8
SELECT up.university, df.difficult_level, ROUND(COUNT(*) / COUNT(DISTINCT dq.device_id),4) "avg_answer_id" FROM user_profile up LEFT JOIN question_practice_detail dq ON up.device_id = dq.device_id LEFT JOIN question_detail df ON dq.question_id = df.question_id WHERE dq.question_id IS NOT NULL GROUP BY up.university, df.difficult_level
从上一题学会了一个神奇的写法
COUNT(DISTINCT dq.device_id)
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