题解 | #统计每个用户的平均刷题数#
统计每个用户的平均刷题数
https://www.nowcoder.com/practice/f4714f7529404679b7f8909c96299ac4
/*注意内连接是笛卡儿积,不能仅仅依据难度分组,还要以学校分组 */
select
up.university,
qd.difficult_level,
count(qpd.question_id)/count(distinct(qpd.device_id)) avg_answer_cnt
from
question_practice_detail qpd
inner join user_profile up on qpd.device_id = up.device_id
inner join question_detail qd on qpd.question_id = qd.question_id
group by
2,1 having university = '山东大学';