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
join临时表 的方法
select a.university,avg(b.id_count) as avg_answer_cnt
from user_profile a
join (select device_id,count(id) as id_count from question_practice_detail group by device_id) b
on a.device_id = b.device_id
group by a.university order by a.university
