GENERATE UNIQUE IDENTIFIRE BASE ON GUID

sp : 
ALTER PROCEDURE [dbo].[SP_UNO]
  @GUID AS NVARCHAR(255)
AS
BEGIN
	-- routine body goes here, e.g.
	-- SELECT 'Navicat for SQL Server'
	DECLARE @UNO AS NVARCHAR(255)
	SELECT @UNO = LEFT(NEWID() , 8)
	
	IF NOT EXISTS (SELECT UNO FROM TBL_CERT WHERE UNO = @UNO)
	BEGIN
		UPDATE TBL_CERT SET UNO = @UNO WHERE GUID = @GUID
	END
	ELSE
	BEGIN
		EXEC DBO.SP_UNO @GUID
	END 
	
END


TRIGGER AFTER INSERT : 

BEGIN
  -- Type the SQL Here.
	DECLARE @GUID NVARCHAR(255)
	SELECT @GUID = GUID FROM INSERTED
	
	EXEC DBO.SP_UNO @GUID
END