Unions in Derived-Table Expressions

A union clause is allowed within a derived-table expression.

For example, the following query yields the contents of the stor_id and ord_num columns of both the sales and sales_east tables:

select * from
   (select stor_id, ord_num from sales
    union
    select stor_id, ord_num from sales_east)
dt_sales_info

The union of two select operations defines the SQL-derived table dt_sales_info.