The Line Item file contains a record of every item ordered. Since a single transaction can
involve one or more products, each record in the Purchase Order file is associated with
(linked to) one or more records in this file. Notice that the file contains a composite primary
key—Purchase Order Number and Item Number. Both of these attributes are
needed to uniquely define each record in the file. They also provide links to related records
in the Purchase Order and Inventory files.