UPDATE
    SCORES AS SC
SET
    RANKING = SC_2.RK
FROM
    (SELECT
        STUDENT_CODE
        ,SUBJECT_CODE
        ,RANK()OVER(PARTITION BY SUBJECT_CODE ORDER BY SCORE DESC) AS RK
    FROM
        SCORES
     ) SC_2
WHERE
    SC.STUDENT_CODE = SC_2.STUDENT_CODE
AND
    SC.SUBJECT_CODE = SC_2.SUBJECT_CODE
;