<P> The standard example of a complete subtype relationship is the party entity . Given the discriminator PARTY TYPE (which could be individual, partnership, C Corporation, Sub Chapter S Association, Association, Governmental Unit, Quasi-governmental agency) the two subtype entities are PERSON, which contains individual - specific information such as first and last name and date of birth, and ORGANIZATION, which would contain such attributes as the legal name, and organizational hierarchies such as cost centers . </P> <P> When sub-type relationships are rendered in a database, the super-type becomes what is referred to as a base table . The sub-types are considered derived tables, which correspond to weak entities . Referential integrity is enforced via cascading updates and deletes . </P> <P> Consider a database that records customer orders, where an order is for one or more of the items that the enterprise sells . The database would contain a table identifying customers by a customer number (primary key); another identifying the products that can be sold by a product number (primary key); and it would contain a pair of tables describing orders . </P> <P> One of the tables could be called Orders and it would have an order number (primary key) to identify this order uniquely, and would contain a customer number (foreign key) to identify who the products are being sold to, plus other information such as the date and time when the order was placed, how it will be paid for, where it is to be shipped to, and so on . </P>

When is the concept of a weak entity used in data modeling