Here is an example of a request expressed using a subquery: "Which customers placed orders after July 13, 2001?"

SELECT GivenName, Surname
FROM Customers
WHERE EXISTS (
   SELECT *
   FROM SalesOrders
   WHERE ( OrderDate > '2001-07-13' ) AND
         ( Customers.ID = SalesOrders.CustomerID ) );

Here, for each row in the Customers table, the subquery checks if that customer ID corresponds to one that has placed an order after July 13, 2001. If it does, the query extracts the first and last names of that customer from the main table.

The EXISTS test does not use the results of the subquery; it just checks if the subquery produces any rows. So the existence test applied to the following two subqueries return the same results. These are subqueries and cannot be processed on their own, because they refer to the Customers table which is part of the main query, but not part of the subquery.

SELECT *
FROM Customers, SalesOrders
WHERE ( OrderDate > '2001-07-13' ) AND 
      ( Customers.ID = SalesOrders.CustomerID )

SELECT OrderDate
FROM Customers, SalesOrders
WHERE ( OrderDate > '2001-07-13' ) AND 
      ( Customers.ID = SalesOrders.CustomerID );

It does not matter which columns from the SalesOrders table appear in the SELECT statement, though by convention, the "SELECT *" notation is used.

You can reverse the logic of the EXISTS test using the NOT EXISTS form. In this case, the test returns TRUE if the subquery produces no rows, and FALSE otherwise.

You may have noticed that the subquery contains a reference to the ID column from the Customers table. A reference to columns or expressions in the main table(s) is called an outer reference and the subquery is said to be correlated. Conceptually, SQL processes the above query by going through the Customers table, and performing the subquery for each customer. If the order date in the SalesOrders table is after July 13, 2001, and the customer ID in the Customers and SalesOrders tables match, then the first and last names from the Customers table appear. Since the subquery references the main query, the subquery in this section, unlike those from previous sections, returns an error if you attempt to run it by itself.

• Correlated and uncorrelated subqueries