Specifies that duplicate rows should not be eliminated from the
    result table. By default, the UNION operator removes duplicate
    rows.

    The following example returns duplicate rows from the result
    table:

    SQL> SELECT LAST_NAME, SEX FROM EMPLOYEES WHERE LAST_NAME = 'Nash'
    cont> UNION ALL
    cont> SELECT LAST_NAME, SEX FROM EMPLOYEES WHERE LAST_NAME = 'Lapointe';
     LAST_NAME        SEX
     Nash             M
     Nash             M
     Lapointe         F
     Lapointe         F
    4 rows selected