tailieunhanh - Lecture Fundamentals of database systems: Chapter 4 - Emasri, Navathe

In this chapter, we describing features that have been proposed for semantic data models, and show how the ER model can be enhanced to include these concepts, leading to the Enhanced ER (EER) model. | EER stands for Enhanced ER or Extended ER EER Model Concepts Includes all modeling concepts of basic ER Additional concepts subclasses superclasses specialization generalization categories UNION types attribute and relationship inheritance These are fundamental to conceptual modeling The additional EER concepts are used to model applications more completely and more accurately EER includes some object-oriented concepts such as inheritance An entity type may have additional meaningful subgroupings of its entities Example EMPLOYEE may be further grouped into SECRETARY ENGINEER TECHNICIAN . Based on the employee s Job MANAGER employees who are managers SALARIED_EMPLOYEE HoUrLY_EMPLOYEE Based on the employee s method of pay EER diagrams extend ER diagrams to represent these additional subgroupings called subclasses or subtypes Copyright 2007 Ramez Elmasri and Shamkant B. Navathe Slide 4-3 Copyright 2007 Ramez Elmasri and Shamkant B. Navathe lide 4-4 Copyright 2007 RamezElmasri and Shamkant B. Navathe lide 4- Each of these subgroupings is a subset of EMPLOYEE entities Each is called a subclass of EMPLOYEE EMPLOYEE is the superclass for each of these subclasses These are called superclass subclass relationships EMPLOYEE SECRETARY EMPLOYEE TECHNICIAN EMPLOYEE MANAGER Copyright 2007 RamezElmasri and Shamkant B. Navathe lide 4- 1 These are also called IS-A relationships . SECRETARY IS-A EMPLOYEE TECHNICIAN IS-A EMPLOYEE . Note An entity that is member of a subclass represents the same real-world entity as some member of the superclass The subclass member is the same entity in a distinct specific role An entity cannot exist in the database merely by being a member of a subclass it must also be a member of the superclass A member of the superclass can be optionally included as a member of any number of its subclasses Examples A salaried employee who is also an engineer belongs to the two subclasses ENGINEER and SALARIED_EMPLOYEE A salaried employee who is also an engineering