Come combinare due tabelle in queste stesse colonne?

SELECT  ID, SUM(Amount) total
FROM
        (
            SELECT ID, Amount FROM A
            UNION ALL
            SELECT ID, AMount FROM B
        ) s
GROUP   BY ID

• Demo SQLFiddle

Puoi creare una tabella di base sul risultato della query.

CREATE TABLE C
AS
SELECT  ID, SUM(Amount) total
FROM
        (
            SELECT ID, Amount FROM A
            UNION ALL
            SELECT ID, AMount FROM B
        ) s
GROUP   BY ID;

• Demo SQLFiddle