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
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Code for TallyGenerator