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 u.university,qd.difficult_level,
round(count(qpd.device_id)/(count(distinct qpd.device_id)),4)
as avg_answer_cnt
from (user_profile u inner join
question_practice_detail qpd on u.device_id = qpd.device_id)
inner join question_detail qd on qpd.question_id = qd.question_id
group by u.university,qd.difficult_level;
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