A query can use numerous nested derived-table expressions, which are SQL expressions that define a SQL-derived table.

In the following example, the innermost derived-table expression defines SQL-derived table dt_1, the select from forming the derived-table expression that defines SQL-derived table dt_2.
select postalcode
   from (select postalcode
            from (select postalcode 
                    from authors) dt_1) dt_2

The degree of nesting is limited to 25.