DECLARE @x XML
SET @x = '<i>' + REPLACE( @IDString, ',', '</i><i>') + '</i>'

SELECT *
FROM MyTable a
	INNER JOIN 
	(SELECT x.i.value('.', 'VARCHAR(7)')
	 FROM @x.nodes('//i') x(i)) b
ON a.ID = b.ID

DECLARE @IDString VARCHAR(MAX)
SET @IDString =
(SELECT (ID) + ','
FROM MYTABLE
--WHERE FILTER CAN GO HERE
FOR XML PATH('')) 

select * from (SELECT x.i.value('.', 'VARCHAR(7)')	FROM @x.nodes('//i') x(i)) b

should read

select * from (SELECT x.i.value('.', 'VARCHAR(7)') as ID	FROM @x.nodes('//i') x(i)) b

DECLARE @IDString VARCHAR(MAX)
SET @IDString =
(SELECT (ID) + ','
FROM MYTABLE
--WHERE FILTER CAN GO HERE
FOR XML PATH('')) 

DECLARE @IDString VARCHAR(MAX)
SET @IDString =
(SELECT
    CASE row_number() OVER(ORDER BY ID)
    WHEN 1 THEN '' 
    ELSE ',' 
    END + (ID)
FROM MYTABLE
--WHERE FILTER CAN GO HERE
FOR XML PATH(''))