Qualifies and alters the result returned from the joined tables.
    There are several types of qualified joins:

    o  INNER JOIN

    o  LEFT OUTER JOIN

    o  RIGHT OUTER JOIN

    o  FULL OUTER JOIN

    o  NATURAL JOIN

    For an INNER and OUTER JOIN, the result table is the combination
    of all columns of the first table reference to all the columns in
    the second table reference. For a NATURAL JOIN, the result table
    condenses common columns (that is, columns with the same name)
    between the table references.