SELECT TEST.DATE, TEST.CODE, SUM(TEST.ERROR_1) AS SUM_ERROR1, SUM(TEST.ERROR_2) AS SUM_ERROR_2 FROM
(
SELECT DATE, CODE, ERROR_1, ERROR_2 FROM A
UNION ALL
SELECT DATE, CODE, ERROR_1, ERROR_2 FROM B
UNION ALL
SELECT DATE. CODE, ERROR_1, ERROR_2 FROM C
) AS TEST
GROUP BY TEST.DATE, TEST.CODE
ORDER BY TEST.DATE;