题解 | #分组过滤练习题#
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 university,count(question_id)/ count(distinct up.device_id) as avga from user_profile as up,question_practice_detail as qpd where up.device_id = qpd.device_id group by university