63
Subqueries Returning Relations
 SELECT  Company.name
 FROM      Company, Product
 WHERE   Company.name=Product.maker
              AND  Product.name  IN
                         (SELECT Purchase.product
                           FROM   Purchase
                           WHERE Purchase .buyer = ‘Joe Blow‘);
Find companies who manufacture products bought by Joe Blow.
Here the subquery returns a set of values: no more
runtime errors.