UPDATE TOTAL_SCORES AS TS
SET TOTAL_SCORE = SUB.TOTAL_SCORE
FROM (
SELECT ST.STUDENT_CODE AS STUDENT_CODE, SUM(SCORE) AS TOTAL_SCORE
FROM STUDENTS ST
JOIN SCORES SC
ON ST.STUDENT_CODE = SC.STUDENT_CODE
GROUP BY ST.STUDENT_CODE
) AS SUB
WHERE TS.STUDENT_CODE = SUB.STUDENT_CODE;