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Subqueries Returning Relations
 SELECT  name
 FROM     Product
 WHERE  price >  ALL (SELECT price
                                        FROM     Purchase
                                        WHERE  maker=‘Gizmo-Works’)
Product ( pname,  price, category, maker)
Find products that are more expensive than all those produced
By “Gizmo-Works”
You can also use:   s > ALL R
                               s > ANY R
                               EXISTS R