题解 | 统计每个学校的答过题的用户的平均答题数

SELECT user_profile.university,
round(count(question_practice_detail.question_id)/count(DISTINCT question_practice_detail.device_id),4) as avg_answer_cnt
from user_profile
inner join  question_practice_detail
on user_profile.device_id =  question_practice_detail.device_id
group by university
order by university;

两个表按照device_id 取交集