1 Database Intro

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Unformatted text preview: ther entity. It can be an arbitrary many value or fixed number be an arbitrary "many" value or a fixed number. • The number of instances of one entity number of instances of one entity that can or must be associated with each instance of another entity. • There are three types of maximum cardinality: – One-to-One [1:1] • Maximum and Minimum Cardinality – One-to-Many [1:N] [1:N] – Many-to-Many [N:M] BUSI 335 Copyright © 2009-10 Y.M. Cheung, William Tan and Carson Woo 53 BUSI 335 The Three Types of Maximum Cardinality Cardinality Copyright © 2009-10 Y.M. Cheung, William Tan and Carson Woo 54 Parent and Child Entities and Child Entities • In a one-to-many relationship: – The entity on the one side of the relationship is called the parent entity or just the parent. – The entity on the many side of the relationship is called the child entity or just the child. • In the figure below, EMPLOYEE iis the parent the figure below EMPLOYEE s the parent and COMPUTER is the child: BUSI 335 Copyright © 2009-10 Y.M. Cheung, William Tan and Carson Woo 55 BUSI 335 Copyright © 2009-10 Y.M. Cheung, William Tan and Carson Woo 56 HAS Relationships HAS-A Relationships Minimum Cardinality Cardinality • The relationships we have been discussing relationships we have been discussing instances of one entity that must be associated with each instance of another entity. are known as HAS-A relationships: – Each entity instance has a relationship with entity instance has with another entity instance: • Minimums are generally stated as either zero or one: • An EMPLOYEE has one or more COMPUTERs. – IF zero [0] THEN participation in the relationship by the entity is optional, and no entity instance must participate in the relationship. – IF one [1] THEN participation in the relationship by the entity is mandatory, and at least one entity instance must participate in the relationship instance must participate in the relationship. • A COMPUTER has an assigned EMPLOYEE....
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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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