DECLARE @Product table(ProductID int)

DECLARE @ProductRelation TABLE (FirstProductID int,SecondProductID int)

INSERT INTO @Product 

SELECT 1
UNION ALL
SELECT 2
UNION ALL
SELECT 3
UNION ALL
SELECT 4
UNION ALL
SELECT 5
UNION ALL
SELECT 6
UNION ALL
SELECT 7
UNION ALL
SELECT 8
UNION ALL
SELECT 9
UNION ALL
SELECT 10

--SELECT * FROM @Product

INSERT INTO @ProductRelation

SELECT 1,2
UNION ALL
SELECT 3,5
UNION ALL
SELECT 2,6
UNION ALL
SELECT 1,4
UNION ALL
SELECT 1,4

--SELECT * FROM @ProductRelation


 
SELECT ProductID,'Not Linked' AS 'Relation' FROM @Product
UNION
SELECT FirstProductID,'Linked' from @ProductRelation
UNION
SELECT SecondProductID ,'Linked' FROM @ProductRelation

ProductID	       Relation

1			Linked
2			Linked
3			Linked
4			Linked
5			Linked
6			Linked
7			Not Linked
8			Not Linked
9			Not Linked
10			Not Linked

SELECT 
	p.ProductID, 
	Relation = CASE x.n WHEN 1 THEN 'Linked' ELSE 'Not Linked' END
FROM @Product p
OUTER APPLY (
	SELECT TOP 1 n=1 
	FROM @ProductRelation r 
	WHERE p.ProductID IN (r.FirstProductID, r.SecondProductID)
) x
ORDER BY p.ProductID

SELECT 
	p.ProductID, 
	Relation = CASE x.n WHEN 1 THEN 'Linked' ELSE 'Not Linked' END
FROM @Product p
OUTER APPLY (
	SELECT TOP 1 n=1 
	FROM @ProductRelation r 
	WHERE p.ProductID IN (r.FirstProductID, r.SecondProductID)
) x
ORDER BY p.ProductID

SELECT
    p.ProductID, CASE WHEN EXISTS(SELECT 1 FROM @ProductRelation pr 
        WHERE p.ProductID IN (pr.FirstProductID, pr.SecondProductID)) 
        THEN 'Linked' ELSE 'Not Linked' END
FROM @Product p
ORDER BY
    p.ProductID

SELECT p.ProductID, 
	CASE WHEN R1.FirstProductID IS NULL and R2.SecondProductID IS NULL then 'Not Linked' ELSE 'Linked'  END Relation 
FROM @Product p
	LEFT JOIN @ProductRelation R1 ON R1.FirstProductID = p.ProductID
	LEFT JOIN @ProductRelation R2 ON R2.SecondProductID= p.ProductID
ORDER BY p.ProductID

SELECT p.ProductID, 
	CASE WHEN R.FirstProductID IS NULL and R.SecondProductID IS NULL then 'Not Linked' ELSE 'Linked'  END Relation 
FROM @ProductRelation R 
	RIGHT JOIN @Product p ON  p.ProductID =  R.FirstProductID OR p.ProductID = R.SecondProductID
ORDER BY p.ProductID