Using select expressions, you can define four types of result
    tables:

    o  Simple tables

    o  Aggregate tables

    o  Joined tables

    o  Derived tables

    A simple table result is generated from a single table or view
    and usually includes the special DBKEY column. The rows of such
    tables can be updated, if privileges allow.

    An aggregate table is a virtual table formed by the GROUP
    BY clause (allowing multi-row results) or an aggregate (or
    statistical) expression without allowing a GROUP BY (a single
    row result).

    A table reference is a base table, view, derived table, or a
    joined table.

    A derived table is a named virtual table that represents data
    obtained through the evaluation of a select expression. A derived
    table is named by the specified correlation name. References to a
    derived table and its columns can be made within the query using
    the correlation name. A derived table is similar to a view in
    that a view is also a virtual table represented by the select
    expression used to define it. Therefore, a derived table is like
    a view whose definition is specified within the FROM clause.

    A joined table is a virtual table that represents data obtained
    through the joining of two table references. The type of join
    between the two table references can be either CROSS, INNER
    JOIN, LEFT OUTER JOIN, RIGHT OUTER JOIN, or FULL OUTER JOIN.
    You need to use the joined table syntax to specify an outer join
    operation.

    There are two types of joined tables:

    o  Qualified join

    o  Cross join