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Databases Illuminated, 4th Edition
book

Databases Illuminated, 4th Edition

by Catherine M. Ricardo, Susan D. Urban, Karen C. Davis
March 2022
Intermediate to advanced content levelIntermediate to advanced
682 pages
22h 58m
English
Jones & Bartlett Learning
Content preview from Databases Illuminated, 4th Edition

3.7 Existence Dependency and Weak Entities

At times we need to store data about an entity that would not be in the database unless we had a related entity already in the database. For example, we would not need to store data about a sales order unless we had a customer. An existence constraint, or existence dependency, can occur between two entities. If X and Y are entities and each instance of Y must have a corresponding instance of X, then we say that Y is existence dependent on X. This means a Y entity cannot exist without some X entity.

If Y is existence dependent on X, then Y must have total participation in its relationship set with X. A special type of existence dependency occurs when the dependent entity set does not have a primary ...

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Publisher Resources

ISBN: 9781284231595