SELECT CS1.Code Code1, CS2.Code Code2
FROM Computer_System CS1, Computer_System CS2
WHERE
(NOT EXISTS
(
        (SELECT Software_Code
        FROM Software_Computer_System WHERE Software_Computer_System.CS_Code=CS1.Code
        MINUS
        SELECT Software_Code
        FROM Software_Computer_System WHERE Software_Computer_System.CS_Code=CS2.Code)
    UNION
        (SELECT Software_Code
        FROM Software_Computer_System WHERE Software_Computer_System.CS_Code=CS2.Code
        MINUS
        SELECT Software_Code
        FROM Software_Computer_System WHERE Software_Computer_System.CS_Code=CS1.Code)
)) AND CS2.Code>CS1.Code;