题解 | #统计每个学校各难度的用户平均刷题数#

select university,qd.difficult_level,round(count(*)/count(distinct u.device_id),4)
from user_profile as u inner join question_practice_detail as qp inner join question_detail as qd 
on u.device_id = qp.device_id and qp.question_id = qd.question_id
group by u.university,qd.difficult_level
# 分组后只能对组内数据进行查看，比如组内的个数或者平均数等
# 这里可以对分组后，再分组，类似于组内拆分