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(b.question_id)/count(distinct(b.device_id)),4) as avg_answer_cnt from user_profile a
left join question_practice_detail b
on a.device_id = b.device_id
left join question_detail c 
on b.question_id = c.question_id
group by university,difficult_level
having difficult_level is not null

(count(b.question_id)/count(distinct(b.device_id)) 分子只能是b. 但是分母可以是a. 也可以是b.

分子分母中的b.不能不写