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 university, detail.difficult_level, count(detail.question_id)/count(distinct detail.device_id)
from
(select qpd.device_id ,qd.question_id, difficult_level
from
question_detail qd, question_practice_detail qpd
where qd.question_id = qpd.question_id) as detail,
user_profile
where detail.device_id = user_profile.device_id
group by university, difficult_level;
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