题解 | #统计每个学校各难度的用户平均刷题数#
统计每个学校各难度的用户平均刷题数
http://www.nowcoder.com/practice/5400df085a034f88b2e17941ab338ee8
等值连接
university,
difficult_level,
count(qpd.question_id)/count(distinct qpd.device_id) as avg_answer
from
question_practice_detail as qpd
,user_profile as up
,question_detail as qd
where qd.question_id = qpd.question_id
and up.device_id = qpd.device_id
group by university, difficult_level