The NULL keyword specifies the null value. When assigned to a
    column of any data type, the NULL keyword forces the column to
    be set to null because the NULL keyword has no data type. For
    example:

    SQL> -- List all employees in the database and all potential employees
    SQL> --
    SQL> SELECT employee_id, last_name, first_name
    cont> FROM employees
    cont> UNION
    cont> SELECT NULL, last_name, first_name
    cont> FROM candidates;
       .
       .
       .
     00418         Blount           Peter
     00435         MacDonald        Johanna
     00471         Herbener         James
     NULL          Boswick          Fred
     NULL          Schwartz         Trixie
     NULL          Wilson           Oscar
    103 rows selected

    The NULL keyword is distinct from the IS NULL predicate which
    tests for null values of an expression. When testing an
    expression for null, using equality with NULL (for example, a
    predicate when testing an expression as shown in the following
    example:

    SQL> SELECT e.last_name, j.job_end
    cont> FROM employees e, job_history j
    cont> WHERE e.employee_id = j.employee_id
    cont> AND j.job_end IS NULL;
     E.LAST_NAME      J.JOB_END
     Smith            NULL
     O'Sullivan       NULL
     Hastings         NULL
       .
       .
       .

    In some cases, the NULL keyword may have a data type of CHAR(31)
    when used in a query that requires a data type; such as in
    arithmetic and character expressions. In such queries, the
    assumption of the CHAR data type may cause an incompatibility
    error. If this occurs, use the CAST function (for example, CAST
    (NULL AS data-type)) to change NULL to a compatible data type for
    the query.