chapter 3 - CGS 2545: Database Concepts Spring 2012 Chapter...

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CGS 2545: Database Concepts (Chapter 3) Page 1 © Dr. Mark Llewellyn CGS 2545: Database Concepts Spring 2012 Chapter 3 – The Enhanced ER Model And Business Rules Department of Electrical Engineering and Computer Science Computer Science Division University of Central Florida Instructor : Dr. Mark Llewellyn markl@cs.ucf.edu HEC 236, 407-823-2790 http://www.cs.ucf.edu/courses/cgs2545/spr2012
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CGS 2545: Database Concepts (Chapter 3) Page 2 © Dr. Mark Llewellyn Introduction The basic ER model was introduce in the 1970s. It has been widely accepted as a suitable model for most business situations. However, the business environment has changed substantially in the 30 plus years that have elapsed since the introduction of the model. Business relationships are now more complex, and as a result, business data is much more complex. To cope with these changes the ER model has been enhanced in many areas since it was first introduced so that it can more accurately represent the complex data encountered in today’s business environment. The term enhanced entity-relationship (EER) model is used to identify the model that has results from extending the original ER model with new modeling constructs. Two main extensions to the ER model incorporated in the EER model are: (1) supertype/subtype relationships, and (2) entity clustering techniques.
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CGS 2545: Database Concepts (Chapter 3) Page 3 © Dr. Mark Llewellyn Supertypes and Subtypes Supertype: An generic entity type that has a relationship with one or more subtypes. Subtype: A subgrouping of the entities in an entity type which has attributes that are distinct from those in other subgroupings. Attribute Inheritance: Subtype entities inherit values of all attributes of the supertype. An instance of a subtype is also an instance of the supertype. The next page illustrates the supertype/subtype hierarchy.
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CGS 2545: Database Concepts (Chapter 3) Page 4 © Dr. Mark Llewellyn . . . attributes of the supertype are shared by all subtypes, including the identifier Supertype attributes unique to subtype 1 Subtype 1 attributes unique to subtype 2 Subtype 2 attributes unique to subtype N Subtype N Relationships in which all class participate Relationships in which only a specific subtype participates Specialized versions of the supertype A general entity type
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(Chapter 3) Page 5 © Dr. Mark Llewellyn Relationships and Subtypes Relationships at the supertype level indicate that all subtypes will participate in the relationship. The instances of a subtype may participate in a relationship unique to that subtype. In this situation, the relationship is shown at the subtype level. An example is shown on the next page.
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chapter 3 - CGS 2545: Database Concepts Spring 2012 Chapter...

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