注意多表查询出来的数据还是根据笛卡尔积计算后一行一行的, 只不过关系是一对多, 所以导致我们要给去重
select university, (count(q.device_id) / count(distinct(u.device_id))) from user_profile as u inner join question_practice_detail as q on u.device_id = q.device_id group by university;