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Unformatted text preview: . . Winter 2008 CPE/CSC 366: Database Modeling, Design and Implementation Alexander Dekhtyar . . The Entity-Relationship Model Aggregation and Class Hierarchies Aggregation Aggregation: Participation of a relationship set in another relationship set . Example. We continue modifying the Library database. Sometimes, library patrons do not return books on time. In such cases, the library assesses late fees (fines) for the books on-loan. Fees are assessed for each day the book is late, and each assessment needs to be recorded separately, together with the assessed amount. To model this, we create (see Figure 1) a new entity set, Fines , which has two attrbutes, Date Assessed and Amount . We note that entities from this entity set are associated neither with just the books, nor with just memberships, but rather with the acts of borrowing books by patrons . These acts are modeled by the Loans relationship set. We use aggregation to make Loans be ”the other end” of the Causes relationship set, which associates the fines with their causes. We also note that Fines is a weak entity set , and that the aggregation of Loans serves as its identifying owner , with Date Assessed being the discriminator ....
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This note was uploaded on 05/19/2008 for the course CSC 365 taught by Professor Dekhtyar during the Spring '08 term at Cal Poly.
- Spring '08