4+-+ER+Modeling

4+-+ER+Modeling - ER Modeling Douglas Havelka Miami...

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Unformatted text preview: ER Modeling Douglas Havelka Miami University ER models represent the conceptual database as viewed by end users Captures the main requirements for a database implementation: Entities, Attributes, and relationships Entities Real world objects Set of objects (object type) Represented by a rectangle Name is a noun Attributes Characteristics or properties of objects Note: we will use crows feet notation Attributes are displayed as a list in a listbox beneath the entity name Required v optional attributes Must have value (in bold) Domain Set of possible valid values GPA (0, 4) Gender (M or F) May re-use, e.g. address Identifiers Primary keys Uniquely identify each instance Underlined in listbox Using schema: CAR(CarVIN , CarModel, CarManf, CarYear, etc.) Composite key Underline all key attributes Composite Attributes Can be broken up into smaller logical parts Address = Street + City + State + Zip Simple Attributes Cannot be broken up Age, gender DESIGN RULE: Always break up! Multi-valued Attribute An attribute that can legitimately have multiple values EmployeeDegrees = AA, BS, MBA, PhD PetColor = Brown and White ContactPhoneNumber = DESIGN RULE: Eliminate! (Why?) Handling Multi-valued Attributes If the MVA can be broken up into meaningful simple attributes, do it Usually this implies the attribute was actually a composite attribute CarColor = CarTopColor, CarBodyColor, CarTrimColor...
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This note was uploaded on 04/01/2012 for the course MIS 245 taught by Professor Havelka during the Spring '12 term at Miami University.

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4+-+ER+Modeling - ER Modeling Douglas Havelka Miami...

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