A union clause is allowed in a subquery inside a derived-table
expression.
The following example uses a union clause in a subquery within a
SQL-derived table to list the titles of books sold at stores listed in the
sales and sales_east
tables:
select title_id from salesdetail
where stor_id in
(select stor_id from
(select stor_id from sales
union
select stor_id from sales_east)
dt_stores)