10.4. Derived Identifiers

When an identifier in one entity includes a foreign key to another entity, we call it a derived identifier. Because the entity containing the derived identifier depends upon another entity for its identity, we call the first the dependent entity. The entity that it depends upon is the target of a many-to-one or one-to-one relationship from the dependent entity, and is called the parent entity. Figure 10-3 shows an example of a data model for the two kinds of entities, with DEPARTMENT table representing the parent entity and PROJECT table representing the dependent entity. Note that in this example there is an additional name primary key column in PROJECT, meaning that the corresponding Project entity has an identifier attribute that is not part of its relationship to Department.

Figure 10-3. Dependent and parent entity tables.