题解 | #统计每个学校各难度的用户平均刷题数#

select university,difficult_level,round(count(qpd.question_id)/count(distinct qpd.device_id),4) avg_answer_cnt
from user_profile up,question_practice_detail qpd,question_detail qd
where up.device_id=qpd.device_id and qpd.question_id=qd.question_id
group by university,difficult_level

发现多表连接查询的没有on条件也能运行，好像如下图所示，有了join语句出现时不能用where语句过滤条件。

select university,difficult_level,round(count(qpd.question_id)/count(distinct qpd.device_id),4) avg_answer_cnt
from user_profile up
left join question_practice_detail qpd
where up.device_id=qpd.device_id
left join question_detail qd 
where qpd.question_id=qd.question_id
group by university,difficult_level