M cheung william tan and carson woo 39 busi 335

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Unformatted text preview: ies The Entity Class and Two Entity Instances Entity Class and Two Entity Instances • Entity: A person, object, event, or concept in the user environment about which the organization wishes to environment about which the organization wishes to maintain data, e.g. – Person : STUDENT, CUSTOMER – Object : PRODUCT, BUILDING – Event : SALE, REGISTRATION – Concept : ACCOUNT, COURSE ACCOUNT COURSE • Entity class — a collection of entities of a given type • Entity instance — the occurrence of a particular instance occurrence of particular entity • There are usually many instances of an entity in an entity class. BUSI 335 Copyright © 2009-10 Y.M. Cheung, William Tan and Carson Woo 41 BUSI 335 Copyright © 2009-10 Y.M. Cheung, William Tan and Carson Woo 42 EMPLOYEE: Attributes in Ellipses in Ellipses Attributes • Attribute: A property or characteristic of an property or characteristic of an entity, e.g. – CUSTOMER : CustomerNo, Name, Address • All entity instances of a given entity class have the same attributes, but vary in the values of the same attributes, but vary in the values of those attributes. • Originally shown in data models as ellipses. shown in data models as ellipses • Data modeling products today commonly show attributes in rectangular form attributes in rectangular form. BUSI 335 Copyright © 2009-10 Y.M. Cheung, William Tan and Carson Woo 43 BUSI 335 Copyright © 2009-10 Y.M. Cheung, William Tan and Carson Woo 44 EMPLOYEE: EMPLOYEE: Attributes in Entity Rectangle in Entity Rectangle Simple vs. Composite Attribute vs Composite Attribute • Simple Attribute: an attribute that cannot be broken down into smaller components, e.g. – VehicleID, Color, Weight and Horsepower of AUTOMOBILE. • Composite Attribute: an attribute that can be broken down into component parts be broken down into component parts, e.g. – Address can be broken down into StreetAddress, City, Province, Country, PostalCode. BUSI 335 Copyright © 2009-10 Y.M. Cheung, William Tan and Carson Woo 45 BUSI 335 Copyright © 2009...
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This document was uploaded on 03/04/2014 for the course COMM 335 at The University of British Columbia.

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