如何获得特定主题的特定用户的等级，以获得所有得分确认关系

Question

---------------------------------------------------------------------
   id   |   stid   |   Subject    |   Total_cumulative  | Year     | 
----------------------------------------------------------------------
    1   |     23   |  English     |    40               | 2014/2015
    2   |     1    |  English     |    29               | 2014/2015
    3   |     13   |  Maths       |    40               | 2014/2015
    4   |     4    |  Physics     |    60               | 2014/2015
    5   |     13   |  Commerce    |    40               | 2014/2015
    6   |     1    |  Biology     |    89               | 2014/2015
    7   |     13   |  English     |    29               | 2014/2015
    8   |     13   |  Agric       |    60               | 2014/2015

现在我有一个查询可以获得所有特定学生的成绩，如下所示

SELECT * FROM results where stid='13' AND year='2014/2015' 
ORDER BY subject ASC LIMIT 20

我有以下结果

---------------------------------------------------------------------
   id   |   stid   |   Subject    |   Total_cumulative  | Year     | 
----------------------------------------------------------------------
    3   |     13   |  Maths       |    40               | 2014/2015
    5   |     13   |  Commerce    |    40               | 2014/2015
    7   |     13   |  English     |    89               | 2014/2015
    8   |     13   |  Agric       |    60               | 2014/2015

但我的目标是让每个科目中的特定学生排名，我在我的第一个sql语句的while循环中有一个子查询，如下所示

SELECT rank_number, id, name, total_cumulative 
FROM ( SELECT id, name, total_cumulative, @rank:=@rank+1 AS rank_number FROM    
(SELECT pl.id, pl.name,SUM(en.total_cumulative) AS total_cumulative
FROM students pl JOIN results en ON pl.id = en.stid
WHERE en.subject = '$subject' AND en.year='$year' 
GROUP BY pl.id ORDER BY total_cumulative DESC ) AS rankings, 
(SELECT @rank:=0) AS r ) AS overall_rankings 
WHERE id = '13'
LIMIT 0, 1";

之后我得到以下结果

----------------------------------------------------------------------------
   id   |   stid   |   Subject    |   Total_cumulative  | Year       | Rank  
----------------------------------------------------------------------------
    3   |     13   |  Maths       |    40               | 2014/2015  | 1
    5   |     13   |  Commerce    |    40               | 2014/2015  | 2
    7   |     13   |  English     |    29               | 2014/2015  | 3
    8   |     13   |  Agric       |    60               | 2014/2015  | 1

但我希望查询检查关系并按以下顺序给出结果

----------------------------------------------------------------------------
   id   |   stid   |   Subject    |   Total_cumulative  | Year       | Rank  
----------------------------------------------------------------------------
    3   |     13   |  Maths       |    40               | 2014/2015  | 1
    5   |     13   |  Commerce    |    40               | 2014/2015  | 2
    7   |     13   |  English     |    29               | 2014/2015  | 2
    8   |     13   |  Agric       |    60               | 2014/2015  | 1

请帮帮我，我是mysql编程的新手