UPDATE total_scores AS TS
SET
    total_score = SS.T_score
FROM
(
    SELECT
         test_code
        ,student_code
        ,SUM(score) AS T_score
    FROM scores
    GROUP BY
         test_code
        ,student_code
) AS SS
WHERE TS.test_code = SS.test_code AND TS.student_code = SS.student_code;