This stored proc takes 2 optional params.

CREATE PROCEDURE bm_GetClients
	@CompanyKey		INT = NULL,
	@ClientKey		INT = NULL

AS
BEGIN

	SELECT *
		FROM bm_Clients
		WHERE CompanyKey = ISNULL(@CompanyKey, CompanyKey) OR
			  ClientKey = ISNULL(@ClientKey, ClientKey)

END

I want to pass either, both, or none. I'm running it like this:

DECLARE @CompanyKey	INT
DECLARE @ClientKey	INT
SET @CompanyKey = 2
SET @ClientKey = 5

EXEC bm_GetClients
EXEC bm_GetClients @ClientKey
EXEC bm_GetClients @CompanyKey, @ClientKey 
EXEC bm_GetClients @CompanyKey

In all cases I'm getting back all records. If I change the OR to AND, then the second run returns no records.