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

OR

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