select id
     , name
     , grade_num
  from (
        select user_id
             , sum(real_grade) as grade_num
             , DENSE_RANK() OVER(ORDER BY SUM(real_grade) DESC) AS rk
          from (
                select user_id
                     , case
                           when type='add' then grade_num
                           when type='reduce' then grade_num * (-1)
                        end as real_grade
                  from grade_info
                 ) t1
         group by user_id
        ) t2
       inner join user as u
             on u.id=t2.user_id
  where rk=1
  order by id asc

!!!嵌套循环