题解 | #统计每个用户的平均刷题数#
统计每个用户的平均刷题数
http://www.nowcoder.com/practice/f4714f7529404679b7f8909c96299ac4
select a.university,
c.difficult_level,
count(b.question_id)/count(distinct b.device_id)
from user_profile a
left join question_practice_detail b on a.device_id=b.device_id
left join question_detail c on b.question_id=c.question_id
where a.university="山东大学"
group by c.difficult_level
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