CS
Chapter04EERModel.pptx

All relationships of the entity as a member of the

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All relationships of the entity as a member of the superclass Example: In the previous slide, SECRETARY (as well as TECHNICIAN and ENGINEER) inherit the attributes Name, SSN, …, from EMPLOYEE Every SECRETARY entity will have values for the inherited attributes Enhanced Entity-Relationship Modelling 9
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Specialization Specialization is the process of defining a set of subclasses of a superclass The set of subclasses is based upon some distinguishing characteristics of the entities in the superclass Example: {SECRETARY, ENGINEER, TECHNICIAN} is a specialization of EMPLOYEE based upon job type. Example: MANAGER is a specialization of EMPLOYEE based on the role the employee plays May have several specializations of the same superclass Enhanced Entity-Relationship Modelling 10
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Specialization Example: Another specialization of EMPLOYEE based on method of pay is {SALARIED_EMPLOYEE, HOURLY_EMPLOYEE}. Superclass/subclass relationships and specialization can be diagrammatically represented in EER diagrams Attributes of a subclass are called specific or local attributes. For example, the attribute TypingSpeed of SECRETARY The subclass can also participate in specific relationship types. For example, a relationship BELONGS_TO of HOURLY_EMPLOYEE Enhanced Entity-Relationship Modelling 11
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Specialization Enhanced Entity-Relationship Modelling 12
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Generalization Generalization is the reverse of the specialization process Several classes with common features are generalized into a superclass; original classes become its subclasses Example: CAR, TRUCK generalized into VEHICLE; both CAR, TRUCK become subclasses of the superclass VEHICLE. We can view {CAR, TRUCK} as a specialization of VEHICLE Alternatively, we can view VEHICLE as a generalization of CAR and TRUCK Enhanced Entity-Relationship Modelling 13
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Generalization Enhanced Entity-Relationship Modelling 14
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Generalization and Specialization Diagrammatic notations are sometimes used to distinguish between generalization and specialization Arrow pointing to the generalized superclass represents a generalization Arrows pointing to the specialized subclasses represent a specialization We do not use this notation because it is often subjective as to which process is more appropriate for a particular situation We advocate not drawing any arrows Enhanced Entity-Relationship Modelling 15
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Generalization and Specialization Data Modeling with Specialization and Generalization A superclass or subclass represents a collection (or set or grouping) of entities It also represents a particular type of entity Shown in rectangles in EER diagrams (as are entity types) We can call all entity types (and their corresponding collections) classes , whether they are entity types, superclasses, or subclasses Enhanced Entity-Relationship Modelling 16
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Types of Specialization Predicate-defined ( or condition-defined) : based on some predicate.
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  • Fall '09
  • SUNANHAN
  • Entity-relationship model

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