Php Mysql学生排名

Question

我在我的数据库中有这些数据，我想要做的就是根据总分，科目，会话和学期获得班级中的学生排名和关系 我试过这个，但得到了不同的立场

SELECT  d.username, c.ranks FROM
     (   SELECT   total_score, @rank:=@rank+1 Ranks   FROM 
          (SELECT  DISTINCT total_score FROM
                   cbt_result a  ORDER BY total_score DESC) t,    
          (SELECT @rank:= 0) r )
          c  INNER JOIN cbt_result d 
          ON c.total_score = d.total_score 
     WHERE term = 'first'  
AND  class_id ='$key' 
AND subject_id = '$subkey' 
AND username ='$student_username' 
AND session_term = '$session_term_key'