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
这道题，直白地说，就是求每个学校、每一种难度的题目，平均被多少作答过。
题目的平均被作答人数=答题的总次数/独立答题人数=count(qpd.question_id)/count(DISTINCT qpd.device_id)
题目的平均被作答人数=答题的总次数/独立答题人数=count(qpd.question_id)/count(DISTINCT qpd.device_id)
SELECT up.university, qd.difficult_level, count(qpd.question_id)/count(DISTINCT qpd.device_id) as avg_answer_cnt FROM question_practice_detail as qpd JOIN user_profile as up ON up.device_id = qpd.device_id JOIN question_detail as qd ON qpd.question_id = qd.question_id GROUP BY up.university, qd.difficult_level;
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