题解 | #统计每个学校的答过题的用户的平均答题数#

select `university`,
round(count(`qpd`.`device_id`)/count(distinct `up`.`device_id`),4)
from `question_practice_detail` as `qpd`
join `user_profile` as `up`
on `qpd`.`device_id` = `up`.`device_id`
group by `university`;