Avoiding Pitfalls in Data Modeling - High-Performance Web Databases: Design, Development, and Deployment - page 283

Exhibit 18-8. Omission and inclusion of supertype.

that any Employee occurrence must be the parent to the associative entity
Project Assignment.

Entity Role in Data Modeling

A role entity is an entity playing a role that is a generalization of the role
played by several other entity types. In Exhibit 9a, the Organization Unit
entity serves as a role entity.

A modeler may encounter situations in which confu-
sion exists about whether to use the more generic role entity instead of
specific entities. Referring to the examples in Exhibit 9, the modeler has the
choice of modeling as in Exhibit 9b with existing concrete business terms
(e.g., division, regional office, or department) or of using a more generic,
more abstract entity named Organizational Unit, as in Exhibit 9a. The role
entity form should be used when a structure is not very stable (e.g., the
regional office is removed), when the structure contains too many levels,
or when the number of levels is undeterminable.

Use of Role Entity.

The point is that even a model appears to reflect the business rules
because it does not show all the business facts that are needed. The pitfall