Entity-Relationship Modeling
Entity-relationship (ER) modeling is a simple and clear method of expressing the design of database. ER modeling isn't new-it was first proposed by Chen in 1976-but it has only emerged as the dominant modeling paradigm for databases in the past 10 or 12 years.
Figure C-2 shows a partial model of the winestore. In this diagram, you can see the relationship between wines, wineries, and regions. Each wine has attributes such as a name, type, and a description. A wine is made by a winery, and each winery has attributes such as a name, phone, and description. Many wineries are located in a region, where a region has a map and description.
Figure C-2. A simple ER model showing the relationship between wines, wineries, and regions
ER diagrams aren't complicated, and we have already illustrated most of the features of ER modeling in Figure C-2. These features include:
- Rectangles
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Represent entities-that is, objects being modeled. Each entity is labeled with a meaningful title.
- Diamonds
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Represent relationships between entities; a relationship is labeled with a descriptive title that represents how the entities interact.
- Ellipses
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Represent attributes that describe an entity.
- Lines
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Connect entities to relationships. Lines may be without any annotation, be annotated with an M and an N, or annotated with an M and a 1 (or an N and a 1). Annotations indicate the cardinality of the relationship; we discuss cardinality later in this section.
- Lines
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Connect attributes to entities. These lines are never labeled.
Other ER modeling tools include double ellipses, dashed ellipses, and double lines; we use some of these advanced features later in this appendix. Useful references for more advanced ER modeling-and enhanced ER (EER) modeling-are provided in Appendix E.