题解 | #统计每个学校各难度的用户平均刷题数#

select up.university,qd.difficult_level,
    round(count(qpd.question_id) / count(distinct qpd.device_id),4) as "平均刷题数"
from question_practice_detail as qpd
left join user_profile as up 
on qpd.device_id = up.device_id 
left join question_detail as qd 
on qpd.question_id = qd.question_id 
group by up.university,qd.difficult_level

这题有三个表，需要先找我们的主表，question_practice_detail 该表的2.3字段分别对应表1和表

做错的地方

1：不同学校，不同难度，把不同难度给忘记了

2：round函数格式以及运用不熟悉