WITH ranksubq AS (
    SELECT
        student_code
        ,RANK() OVER(ORDER BY total_score DESC) AS rank
    FROM 
        total_scores
    )
    
UPDATE
    total_scores
SET
    ranking = ranksubq.rank
FROM
    ranksubq
WHERE
    total_scores.student_code = ranksubq.student_code;
        
--SELECT * FROM total_scores;