题解 | #统计每个学校的答过题的用户的平均答题数#
统计每个学校的答过题的用户的平均答题数
http://www.nowcoder.com/practice/88aa923a9a674253b861a8fa56bac8e5
select a.university,avg(b.answer_cnt) avg_answer_cnt from user_profile a inner join ( select device_id,count(question_id) answer_cnt from question_practice_detail group by device_id )b on a.device_id=b.device_id group by a.university order by a.university 题目隐含条件：按校名排序
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