This is a very simple scenario, but is taken from a very complex one.
A Customer may have more than 1 address, some may have none. You only want each customer to appear only once. So what can you do?
SELECT C.Name, A.Address
FROM Customer AS C
OUTER APPLY (SELECT TOP 1 *
WHERE CID = C.ID) AS A
Note, that this will never work on SQL CE, but on SQL Server 2005 or 2008! And it’s not part of the ANSI standard!