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Enhanced Entity-Relationship Modeling.pptx
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- 2. 2 Introduction • The EnhancedE-R Model incorporates extensions to Chen's original E- R model. There have been literally dozens of proposed enhancements. We cover the most significant ones in these notes.
- 3. 3 Specialization / Generalization •Superclass • An entity type that includes one or more distinct subgroupings of its occurrences. • Subclass • A distinct subgrouping of occurrences of an entity type.
- 4. 4 Supertypes and Subtypes •Subtype: A subgrouping of the entities in an entity type which has attributes that are distinct from those in other subgroupings • Supertype: An generic entity type that has a relationship with one or more subtypes • Attribute Inheritance: • Subtype entities inherit values of all attributes of the supertype • An instance of a subtype is also an instance of the supertype Sounds like object-oriented?
- 5. 5 GENERALIZATION HIERARCHY --Subtypes and Supertypes. • This is the concept of categorizing or generalizing between supertypes and subtypes of entities. • The basic notation used in McFadden, Hoffer and Prescott is shown in Figure 1. • The Supertype entity is connected to the Subtype entities through lines and a circle. The U-symbol indicates the Subtype is a subset of the Supertype. • All of attributes of the Supertype are also common to the Subtype. • Each Subtype entity has attributes that are unique to that Subtype - that is what makes the entity a subtype. • All entities share a common identifier attribute
- 6. 6 Figure -Basic notationfor supertype/subtype relationships
- 7. 7 Figure – Employeesupertype with three subtypes All employee subtypes will have employee number, name, address, and date-hired Each employee subtype will also have its own attributes
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- 10. 10 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
- 11. 11 Figure – Supertype/subtyperelationships in a hospital Both outpatients and resident patients are cared for by a responsible physician Only resident patients are assigned to a bed
- 12. 12 Generalization and Specialization •Generalization: The process of defining a more general entity type from a set of more specialized entity types. BOTTOM-UP • Specialization: The process of defining one or more subtypes of the supertype, and forming supertype/subtype relationships. TOP-DOWN
- 13. 13 Figure – Exampleof generalization Notice anything? All these types of vehicles have common attributes
- 14. 14 Figure – Generalizationto VEHICLE supertype So we put the shared attributes in a supertype Note: no subtype for motorcycle, since it has no unique attributes
- 15. 15 Figure– Example ofspecialization Only applies to manufactured parts Applies only to purchased parts
- 16. 16 Specialization / Generalization •Superclass/subclass relationship is one-to- one (1:1). • Superclass may contain overlapping or distinct subclasses. • Not all members of a superclass need be a member of a subclass.
- 17. 17 Specialization / Generalization •Attribute Inheritance • An entity in a subclass represents same ‘real world’ object as in superclass, and may possess subclass- specific attributes, as well as those associated with the superclass.
- 18. 18 Specialization / Generalization •Specialization • Process of maximizing differences between members of an entity by identifying their distinguishing characteristics. • Generalization • Process of minimizing differences between entities by identifying their common characteristics.
- 19. 19 AllStaff relation holdingdetails of all staff
- 20. 20 Specialization/generalization of Staff entityinto subclasses representing job roles
- 21. 21 Specialization/generalization of Staff entityinto job roles and contracts of employment
- 22. 22 EER diagram withshared subclass and subclass with its own subclass
- 23. 23 Constraints on Specialization/ Generalization • Two constraints that may apply to a specialization/generalization: • participation constraints • disjoint constraints. • Participation constraint • Determines whether every member in superclass must participate as a member of a subclass. • May be mandatory or optional.
- 24. 24 Constraints on Specialization/ Generalization • Disjoint constraint • Describes relationship between members of the subclasses and indicates whether member of a superclass can be a member of one, or more than one, subclass. • May be disjoint or nondisjoint.
Editor's Notes
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