Adding a SELECT to a Data Flow lets you add a subselect, a select within a select. A subselect, also called a subquery, allows you to use the results of one select as part of another select. While subselects can be used in different places in the SQL language, adding a select to a Data Flow lets you use a select statement as a source, for example, to join to an aggregated or filtered selection from a table. By using a subselect, the need for intermediate staging tables can be avoided.
From the data flow workspace:
Right-click an SQL, JOIN or UNION object, select Add, and then click Select.