题解 | #统计每个用户的平均刷题数#

select university,difficult_level,
round(count(qpd.question_id)/count(distinct(qpd.device_id)),4)

from user_profile as u
inner join question_practice_detail as qpd
on u.device_id = qpd.device_id
inner join question_detail as qd
on qpd.question_id=qd.question_id

where university = '山东大学'
group by qd.difficult_level