SELECT <column-list> FROM <table1> WHERE <column1> IN (SELECT <column2> FROM <table2>)

A subquery is a SELECT statement that return a single column resultset that can be nested inside a SELECT statement or inside another subquery.

Table: Customers

CustID	Name	Address	City	Country	PostalCode	Telephone	Fax
CUST001	Maria Anders	Obere Str. 57	Berlin	Germany	12209	030-0074321	030-0076545
CUST002	Ernst Handel	Kirchgasse 6	Graz	Austria	8010	7675-3425	7675-3426
CUST003	Helen Bennett	Garden House Crowther Way	Cowes	UK	PO31 7PJ	(198) 555-8888	(198) 555-8889
CUST004	Paula Wilson	2817 Milton Dr.	Albuquerque	USA	87110	(505) 555-5939	(505) 555-5940
CUST005	Liu Wong	55 Grizzly Peak Rd.	Butte	USA	59801	(406) 555-5834	(406) 555-8083

Table: Orders

OrderID	Customer	Employee	Order Date	ShippedVia	Freight	ShipCountry
10248	Maria Anders	Buchanan, Steven	04-07-1996	Federal Shipping	$32.38	Germany
10249	Ernst Handel	Peacock, Margaret	08-07-1996	United Package	$65.83	Brazil
10250	Ernst Handel	Leverling, Janet	08-Jul-1996	United Package	$58.17	Brazil
10251	Paula Wilson	Buchanan, Steven	11-Jul-1996	United Package	$22.98	Switzerland
10252	Liu Wong	Dodsworth, Anne	12-Jul-1996	Federal Express	$148.33	Switzerland

SELECT City, Country FROM Customers WHERE Name IN (SELECT DISTINCT Customer FROM Orders WHERE ShippedVia = 'United Package')

The inner SQL statement (SELECT DISTINCT Customer FROM Orders WHERE ShippedVia='United Package') will be execute first. This statement will return 2 records as shown in Resultset1.

Resultset1

Customer

Ernst Handel

Paula Wilson

The returned Resultset1 will be inserted into the IN conditions for the outer SQL statement. The SQL statement will return results in Resultset2.

Resultset2

City	Country
Graz	Austria
Albuquerque	USA

.