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Modeling Objects
In an Entity-Relationship model, distinguishable things are known as entities, each entity is defined by its component attributes, and the affiliations, or relationships, between entities are identified (together, attributes and relationships are known as properties). From these three simple modeling objects, arbitrarily complex systems can be modeled. For instance, a company's customer base, a library of books, or a network of computers can all be depicted as E-R models. If the parts of a system can be identified, the system can be expressed as anE-R model.
Pure Entity-Relationship modeling is independent of native database architecture. Theoretically, an E-R model can be implemented as a relational database, an object-oriented database, a file system, or any other data storage system. In practice, E-R modeling fits most naturally with relational databases; in other words, with databases that store data in two-dimensional tables. The examples and illustrations in this chapter follow this lead by posing a hypothetical relational database server from which data is drawn.
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