Enhanced entity relationship modelling 24 example of

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Enhanced Entity-Relationship Modelling 24
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Example of Disjoint Partial Specialization Enhanced Entity-Relationship Modelling 25
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Example of Overlapping Total Specialization Enhanced Entity-Relationship Modelling 26
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Specialization/Generalization Hierarchies, Lattices & Shared Subclasses A subclass may itself have further subclasses specified on it forms a hierarchy or a lattice Hierarchy has a constraint that every subclass has only one superclass (called single inheritance ); this is basically a tree structure In a lattice , a subclass can be subclass of more than one superclass (called multiple inheritance ) Enhanced Entity-Relationship Modelling 27
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Shared Subclass “Engineering_Manager” Enhanced Entity-Relationship Modelling 28
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Specialization/Generalization Hierarchies, Lattices & Shared Subclasses In a lattice or hierarchy, a subclass inherits attributes not only of its direct superclass, but also of all its predecessor superclasses A subclass with more than one superclass is called a shared subclass (multiple inheritance) Can have: specialization hierarchies or lattices, or generalization hierarchies or lattices, depending on how they were derived We just use specialization (to stand for the end result of either specialization or generalization) Enhanced Entity-Relationship Modelling 29
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Specialization/Generalization Hierarchies, Lattices & Shared Subclasses In specialization , start with an entity type and then define subclasses of the entity type by successive specialization called a top down conceptual refinement process In generalization , start with many entity types and generalize those that have common properties Called a bottom up conceptual synthesis process In practice, a combination of both processes is usually employed Enhanced Entity-Relationship Modelling 30
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Specialization / Generalization Lattice Example (UNIVERSITY) Enhanced Entity-Relationship Modelling 31
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Categories (UNION TYPES) All of the superclass/subclass relationships we have seen thus far have a single superclass A shared subclass is a subclass in: more than one distinct superclass/subclass relationships each relationships has a single superclass shared subclass leads to multiple inheritance In some cases, we need to model a single superclass/subclass relationship with more than one superclass Superclasses can represent different entity types Such a subclass is called a category or UNION TYPE Enhanced Entity-Relationship Modelling 32
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Categories (UNION TYPES) Example: In a database for vehicle registration, a vehicle owner can be a PERSON, a BANK (holding a lien on a vehicle) or a COMPANY. A category (UNION type) called OWNER is created to represent a subset of the union of the three superclasses COMPANY, BANK, and PERSON A category member must exist in at least one (typically just one) of its superclasses Difference from shared subclass , which is a: subset of the intersection of its superclasses shared subclass member must exist in all of its superclasses Enhanced Entity-Relationship Modelling 33
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Two categories (UNION types): OWNER, REGISTERED_VEHICL E Enhanced Entity-Relationship Modelling 34
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Formal Definitions of EER Model Class C:
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  • Fall '09
  • SUNANHAN
  • Entity-relationship model

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