SELECT CS1.Code Code1, CS2.Code Code2
FROM Computer_System CS1, Computer_System CS2
WHERE
(NOT EXISTS
(
        (SELECT Code_Software
        FROM Software_CS WHERE Software_CS.Code_CS=CS1.Code
        MINUS
        SELECT Code_Software
        FROM Software_CS WHERE Software_CS.Code_CS=CS2.Code)
    UNION
        (SELECT Code_Software
        FROM Software_CS WHERE Software_CS.Code_CS=CS2.Code
        MINUS
        SELECT Code_Software
        FROM Software_CS WHERE Software_CS.Code_CS=CS1.Code)
)) AND CS2.Code>CS1.Code;