不能用left join 他会导致最后结果出现None

1 多表关联
2 正确答题/总答题数 3 将数据排序

select qud.difficult_level,sum(if(que1.result="right",1,0))/count(que1.result) as correct_cate  from 

(select
  user.device_id,
  university,
 que.question_id,
 que.result
  
from
  user_profile as user 
  join (
    select
      device_id,
      question_id,
      result
    from
      question_practice_detail
  ) as que on user.device_id = que.device_id  where university = "浙江大学" )
  as que1

    join 
    question_detail 
    as qud on que1.question_id=qud.question_id
    
    group by qud.difficult_level 
    order by correct_cate asc