Or if NULL is a valid parameter to pass to @transaction_guid uniqueidentifier then obtain a GUID value and use that as the default:CREATE PROCEDURE proc_test@cf_user_id nvarchar(100) = NULL,@app_user_id int = NULL,@transaction_guid uniqueidentifier = '10E1BBB9-C48A-4CD1-AA09-D31798361572'ASBEGIN-- SET NOCOUNT ON added to prevent extra result sets from-- interfering with SELECT statements.SET NOCOUNT ON;SET @transaction_guid = COALESCE(NullIf(@transaction_guid, '10E1BBB9-C48A-4CD1-AA09-D31798361572'), newid())...END