Left join with parameter option

Hi, I'm stuck with a fairly simple query (at least I think it might
be).

I need to retreive a selection of case_numbers that match the criteria
set by a parameter value, but also I want to be able to bypass that
part of the criteria if the parameter is '0' (ideally) or Null

The problem is that the table is a Right join.

Is this possible?



Here is my code

DECLARE @DESIG TABLE (state_id varchar(4))
--INSERT INTO @DESIG (state_id)
--SELECT 'PL'
INSERT INTO @DESIG (state_id)
SELECT 'ES'
--INSERT INTO @DESIG (state_id)
--SELECT '0'
--INSERT INTO @DESIG (state_id)
--SELECT 'NULL'


SELECT PAT_REPORT_TABLE.CASE_NUMBER,
DESIGNATED_STATES.STATE_ID,
(Select [dbo].[wr_designations_list] (PAT_CASE.CASE_ID)) as
DESIGNATED_COUNTRYS_LIST
FROM PAT_CASE
INNER JOIN PAT_REPORT_TABLE
ON PAT_CASE.CASE_ID = PAT_REPORT_TABLE.CASE_ID
INNER JOIN STATE_NAME
ON PAT_CASE.STATE_ID = STATE_NAME.STATE_ID
AND STATE_NAME.LANGUAGE_ID = 3
RIGHT OUTER JOIN DESIGNATED_STATES
ON DESIGNATED_STATES.CASE_ID = PAT_CASE.CASE_ID
AND DESIGNATED_STATES.STATE_ID IN (SELECT * FROM @DESIG) ---///
** THIS IS WHERE I WANT TO ADD THE PARAMETER OPTION

WHERE (PAT_CASE.STATE_ID IN ('EP'))
ORDER BY PAT_REPORT_TABLE.CASE_NUMBER




Thanks for any assistance