1.两张小表先关联        
2.再和 user_profile表关联
3.分组  group by university,difficult_level
4.count(question_id)/count(distinct u.device_id)  总的答题数量除答题人数 

select university,difficult_level,count(question_id)/count(distinct u.device_id)
from user_profile u,(select x.*,y.difficult_level
                     from question_practice_detail x,question_detail y
                     where x.question_id = y.question_id) as m
where u.device_id = m.device_id
group by university,difficult_level