题解 | 统计每个用户的平均刷题数

select 
u.university,
qd.difficult_level,
round(count(qpd.question_id)/count(distinct(qpd.device_id)),4)
from user_profile u
join question_practice_detail qpd on u.device_id = qpd.device_id
join question_detail qd on qpd.question_id = qd.question_id
where university like "山东大学"
group by university,difficult_level

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