UPDATE total_scores
SET
  total_score = tmp_total.sum_score
FROM
  (
    SELECT
      student_code,
      SUM(score) AS sum_score
    FROM
      scores
    GROUP BY
      student_code
  ) AS tmp_total
WHERE
  total_scores.student_code = tmp_total.student_code;

SELECT
  *
FROM
  total_scores;