Database: Entity-relationship Model

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Appendix A: A Practical Guide to Entity-Relationship Modeling

A Practical Guide to Entity-Relationship Modeling
Il-Yeol Song and Kristin Froehlich College of Information Science and Technology Drexel University Philadelphia, PA 19104

Abstract
The Entity-Relationship (ER) model and its accompanying ER diagrams are widely used for database design and Systems Analysis. Many books and articles just provide a definition of each modeling component and give examples of pre-built ER diagrams. Beginners in data modeling have a great deal of difficulty learning how to approach a given problem, what questions to ask in order to build a model, what rules to use while constructing an ER diagram, and why one diagram is better than another. In this paper, therefore, we present step-by-step guidelines, a set of decision rules proven to be useful in building ER diagrams, and a case study problem with a preferred answer as well as a set of incorrect diagrams for the problem. The guidelines and decision rules have been successfully used in our beginning Database Management Systems course for the last eight years. The case study will provide readers with a detailed approach to the modeling process and a deeper understanding of data modeling.

Introduction
Entity relationship diagrams (ERD) are widely used in database design and systems analysis to represent systems or problem domains. The ERD was introduced by Chen (1976) in early 1976. Teorey, Yang, and Fry (1986) present an extended ER model for relational database design. The ERD models a given problem in terms of its essential elements and the interactions between those elements in a problem domain. The ERD can serve as the basis for databases, which store data about the problem domain, and which use, manipulate, and constrain that data. Experts in systems analysis and database design are adept at identifying user requirements and then translating them into corresponding components of the model. Many books and articles just provide a definition of each modeling component and give examples of pre-built ER diagrams. Beginners in data modeling have a great deal of difficulty learning how to approach a given problem, what questions to ask in order to build a model, what rules to use while constructing an ER diagram, and why one diagram is better than another.

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Appendix A: A Practical Guide to Entity-Relationship Modeling

Ahrens and Song (1991) present a set of requirements elicitation template sentences, structured English template sentences, and some decision rules for database modeling. This paper presents a set of heuristic rules which improve upon those presented by Ahrens and Song (1991), together with a detailed case study analysis. We include step-by-step guidelines, a set of decision rules proven to be useful in building ER diagrams, and a case study problem with a preferred answer as well as a set of incorrect diagrams for the problem. These guidelines and decision rules have been successfully used in our beginning Database Management Systems course for the last eight years. The case study will provide readers with a detailed approach to the modeling process and a deeper understanding of data modeling.

The Entity-Relationship Diagram
The entity relationship diagram is a graphical representation of a conceptual structure of a problem domain being modeled. The ERD assists the database designer in identifying the data and the rules that will be represented and used in a database. The ERD is an implementation-independent representation of a problem domain and it facilitates communication between the end-user and the analyst. ERDs can be easily converted into a logical database structure that can be readily implemented in a particular commercial database management system. The basic components of the ERD are entities, properties of entities called attributes, and relationships between entities.

Entities
Entities are PRIMARY THINGS of a problem domain about...
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