SELECT DISTINCT Company.name

 FROM      Company, Product, Purchase

 WHERE   Company.name= Product.maker

              AND  Product.name  = Purchase.product

              AND  Purchase.buyer = ‘Joe Blow’

 SELECT DISTINCT Company.name

 FROM      Company, Product

 WHERE   Company.name= Product.maker

              AND  Product.name  IN

                         (SELECT Purchase.product

                           FROM    Purchase

                           WHERE Purchase.buyer = ‘Joe Blow’)

Now

they are

equivalent