Although the basic E-R concepts can model most database features, some aspects of a database may be more aptly expressed by certain extensions to the basic E-R model. In this section, we discuss the extended E-R features of specialization, generalization, higher-and lower-level entity sets, attribute inheritance, and aggregation.
To help with the discussions, we shall use a slightly more elaborate database schema for the university. In particular, we shall model the various people within a university by defining an entity set person, with attributes ID, name, and address. 继续阅读“Database Design ~ Database Design and the E-R Model IV”