CS 157 A Intro to Database Sys Summary

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CS 157 A: Intro to Database Sys Summary of Chapter Section 4. 1 of the textbook Data Modeling and Database Design Copyright 2007 Thomson Course Technology Presentation compiled by Michael Goebel 2007 September 06

Enhanced Entity-Relationship (EER) Modeling: Superclass/Subclass Relationship General Properties of Superclass/Subclass Specialization and Generalization Specialization Hierarchy Specialization Lattice Categorization Appropriate EER Construct Aggregation

Inter-entity class relationship You enter a store to buy a piece of furniture Store and Furniture: different independent object types (aka entity types) Store has Furniture (has-a relationship)

Intra-entity class relationship Need to furnish your new apartment You want a table, chairs, bed, etc. These possess some common properties Table/Chair/Bed is Furniture (is -a relationship)

Superclass and Subclass No relationship type between Furniture and Chair/Bed/Table Furniture can be many different things Furniture is Superclass (SC) Chair/Bed/Table is Subclass (sc)

Basic Properties of SC/sc A Superclass/Subclass relationship One Superclass is related to one or more Subclasses A Subclass entity only belongs to 1 Superclass A Superclass entity can belong to 1 or more (or none) Subclasses Subclass inherits all attributes of Superclass Subclass can have its own attributes Cardinality ratio 1: 1 is understood to exist

New EER diagram notation for SC/sc Total completeness Partial completeness Disjoint Subclass Overlapping Subclasses Union of Superclasses Aggregation of Subclasses Identifies a Subclass

Specialization / Generalization Specialization is the process of discovering the distinguishing attributes of any subgroups and creating Subclasses Specialization is a top-down approach to SC/sc Generalization is the process of gathering all the common attributes shared by a group of entities and creating a Superclass Generalization is a down-up approach to SC/sc

More Specialization Inclusion of a Superclass in the specialization is called a Completeness Constraint can be either Total or Partial Total Specialization: every entity of the Superclass participates in the specialization relationship (shown with a solid line) Partial Specialization: 1 or more entities of the Superclass participates in the specialization relationship (shown with a dash line)

Total/Partial Completeness

More Specialization: Multiple

Predicate How to directly determine the membership of a Subclass Thru the use of a defining predicate Student_Athlete Subclass membership are Sport_Teams

Specialization Hierarchy Already know that a Superclass can have one or more Subclasses Those Subclasses themselves can become Superclasses of other Subclasses The structure of Superclass-Subclass-etc. is limited to an inverted tree Limitation is that a Subclass can only be related to one Superclass specialization

Specialization Lattice Similar to the Specialization Hierarchy where a child (Subclass) is only allowed on parent (Superclass specialization) The child (Subclass) is allowed to have 2 or more parents (Superclass) as long as each parent is from a different Specialization. The Subclass then inherits all the attributes and relationship types from each parent, Multiple Type Inheritance

Categorization The idea that there is only one Superclass is the overriding characteristic of the Specialization/Generalization construct When many Superclasses and a Subclass are tied together by a single relationship type this is known as Categorization The Subclass is known as the Category An entity which belongs to the Category must also exist in only one of the Superclasses

Which EER Construct? When to use Specialization/Generalization or Categorization Not always easy to know: subjective and context specific If Superclasses share many common attributes with same properties then Specialization/Generalization might work best If Superclasses don't share much attributes but have a common category they could identify with, then Categorization might work best

Aggregation (to combine) is similar to Categorization except that it must have Total Completeness Aggregation plays the “is a part of” in a SC/sc relationship

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