Preserves all rows from the left-specified table reference and
    all rows from the right-specified table reference in the result.
    NULL appears in any column that does not have a matching value in
    the corresponding column. For example:

    SQL> SELECT *
    cont> FROM TABLE1 FULL OUTER JOIN TABLE2
    cont> ON TABLE1.C1 = TABLE2.C1;
       TABLE1.C1     TABLE1.C2     TABLE2.C1   TABLE2.C4
              10            15            10   AA
            NULL          NULL            15   BB
              20            25            20   CC
              30            35          NULL   NULL
    4 rows selected

    You must specify at least one equijoin condition in the ON clause
    of a FULL OUTER JOIN clause. This restriction does not apply to
    a FULL OUTER JOIN clause with the USING clause or to the NATURAL
    FULL OUTER JOIN clause.

    An equijoin matches values in columns from one table with the
    corresponding values of columns in another table implicitly using
    an equal (=)  sign.