SQL - Conditional WHERE clause

Question

I have a SQL Server 2005 stored procedure that performs a query. This stored procedure takes in three parameters. The parameters are as follows:

@Stat

Answer 1

SELECT * 
FROM Customer c 
WHERE c.StateID=@StateID 
AND c.CountyID= ISNULL(@CountyID, c.CountyID) 
AND c.CityID = ISNULL(@CityID, c.CityID)