Subqueries allow you to embed one SELECT statement within another. By placing a subquery in a query's criteria, you can base one query on the result of another. Figure 11.38 shows an example. The query pictured finds all the clients without projects. The SQL statement looks like this:
SELECT DISTINCTROW tblClients.ClientID, tblClients.CompanyName FROM tblClients WHERE tblClients.ClientID Not In (Select ClientID from tblProjects)
This query first runs the SELECT statement SELECT ClientID from tblProjects. It uses the result as criteria for the first query.