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

Couple those with some OR statements:

SELECT *
FROM
    Customer c
WHERE
    c.StateID=@StateID
    AND ( c.CountyID=@CountyID OR @CountyID IS NULL )
    AND ( c.CityID=@CityID OR @CityID IS NULL )

For each of those parameters, if it is null, then the check is basically ignored.