I want to compare two strings (containing comma seperated values), after comparing i want only one string without existing values

string a = "1,2,3,4";
string b = "2,3,4,5";

var c = b.Split(',').Except(a.Split(',')).ToArray();

--declare variable to store values in format: '1,2,3,4'
DECLARE @storedValues TABLE(myValues NVARCHAR(30))
DECLARE @selectedValues TABLE(myValues NVARCHAR(30))

INSERT INTO @storedValues (myValues)
VALUES('1,2,3,4')

INSERT INTO @selectedValues (myValues)
VALUES('2,3,4,5')

DECLARE @storedNumbers TABLE(myNumber INT)
DECLARE @selectedNumbers TABLE(myNumber INT)

--use CTE to get stored values 
;WITH stVal AS
(
	SELECT LEFT(myValues, CHARINDEX(',',myValues)-1) AS myVal, RIGHT(myValues, LEN(myValues) - CHARINDEX(',',myValues)) AS Remainder
	FROM @storedValues
	WHERE CHARINDEX(',',myValues)>0
	UNION ALL
	SELECT LEFT(Remainder, CHARINDEX(',',Remainder)-1) AS myVal, RIGHT(Remainder, LEN(Remainder) - CHARINDEX(',',Remainder)) AS Remainder
	FROM stVal
	WHERE CHARINDEX(',',Remainder)>0
	UNION ALL
	SELECT Remainder AS myVal, NULL AS Remainder
	FROM stVal
	WHERE CHARINDEX(',',Remainder)=0
)
INSERT INTO @storedNumbers (myNumber)
SELECT myVal
FROM stVal

--uncheck below lines to check returned values
--SELECT myNumber AS StoredNumbers
--FROM @storedNumbers

--use CTE to get selected values 
;WITH selVal AS
(
	SELECT LEFT(myValues, CHARINDEX(',',myValues)-1) AS myVal, RIGHT(myValues, LEN(myValues) - CHARINDEX(',',myValues)) AS Remainder
	FROM @selectedValues
	WHERE CHARINDEX(',',myValues)>0
	UNION ALL
	SELECT LEFT(Remainder, CHARINDEX(',',Remainder)-1) AS myVal, RIGHT(Remainder, LEN(Remainder) - CHARINDEX(',',Remainder)) AS Remainder
	FROM selVal
	WHERE CHARINDEX(',',Remainder)>0
	UNION ALL
	SELECT Remainder AS myVal, NULL AS Remainder
	FROM selVal
	WHERE CHARINDEX(',',Remainder)=0
)
INSERT INTO @selectedNumbers (myNumber)
SELECT myVal
FROM selVal

--uncheck below lines to check returned values
--SELECT myNumber AS SelectedNumbers
--FROM @selectedNumbers

--below query gets only 5
SELECT myNumber AS DiffNumbers
FROM @selectedNumbers 
WHERE myNumber NOT IN (SELECT myNumber FROM @storedNumbers)

--below query gets only 1
SELECT myNumber AS DiffNumbers
FROM @storedNumbers
WHERE myNumber NOT IN (SELECT myNumber FROM @selectedNumbers)

--to get 1 and 5, use:
SELECT myNumber AS DiffNumbers
FROM @selectedNumbers 
WHERE myNumber NOT IN (SELECT myNumber FROM @storedNumbers)
UNION ALL
SELECT myNumber AS DiffNumbers
FROM @storedNumbers
WHERE myNumber NOT IN (SELECT myNumber FROM @selectedNumbers)