1.分组为：university,difficult_level
2.需将3个表连接：按顺序为a,b,c表，将a表与d表（已连接的b，c表为d表）连接
代码如下： 
select a.university,
    d.difficult_level,
    count(d.question_id)/count(distinct d.device_id) as avg_answer_cnt
from user_profile a
right join (select b.device_id,
                b.question_id,
                b.result,c.difficult_level
           from question_practice_detail b
           left join question_detail c
           on b.question_id=c.question_id
           ) d 
on a.device_id=d.device_id
group by a.university,d.difficult_level;