Syntax options:

    UNION | UNION DISTINCT

    Merges the results of a select expression with another select
    expression into one result table by appending the values of
    columns in one table with the values of columns in other tables.

    The following example extracts the EMPLOYEE_ID of current
    employees with a salary greater than $50,000 and with a Ph.D.
    Duplicate rows are eliminated from the result table:

    SQL> SELECT EMPLOYEE_ID
    cont>   FROM CURRENT_SALARY
    cont>   WHERE SALARY_AMOUNT > 50000
    cont> UNION
    cont>   SELECT EMPLOYEE_ID
    cont>     FROM DEGREES
    cont>     WHERE DEGREE = 'PhD';
     EMPLOYEE_ID
     00164
     00166
     00168
     00169
     00172
     00182
       .
       .
       .
     00418
     00435
     00471
    38 rows selected