ABSTRACT
In the ›rst several chapters of this book, we presented the entity relationship (ER) diagram as a conceptual database tool. ”e approach we took in developing an ER diagram was to assume that we were to model reality for a user. Although we worked on the basics of the ER diagram, there are situations for which the basic model fails to completely describe the reality of the data to be stored. With the increase in the types of database applications, the basic concepts of ER modeling (as originally developed by Chen) were not su±cient to represent the requirements of more complex applications like generalizations and specializations (class hierarchies). An ER model that supports these additional semantic concepts is called the enhanced entity relationship (EER) model (Elmasri and Navathe, 2007). In this chapter, we discuss generalizations and specializations in the EER model and develop a methodology and grammar for this extension. We also discuss shared subclasses and categories or union types. We present a methodology to map the EER diagram to a relational database.
