Weak entity

The foreign key is an attribute of the identifying (or owner, parent, or dominant) entity set.

Two entities can be associated without either being classified as weak, even if one depends on the other, as long as each has its own unique attribute.

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.

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.

For example, as soon as you want to find seasonal or geographical trends in the sales of the item, you need information from the related Order record.