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=@CountyID) OR (@CountyID IS NULL)) AND ((c.CityID=@CityID) OR (@CityID IS NULL))