The Entity-Relationship Model for Database Management
Peter Chen (Chen, 1976) introduced the Entity-Relationship (E-R) model as a data model that was intended to replace the relational model. The E-R model was a serious attempt to extend the semantics of the relational model to capture more meaning from the data stored in a database. There is a naive appeal to the E-R model because it is based on a far less abstract approach to the world than that used by the relational model. In the E-R approach, a real world object is captured as an entity. Each entity has certain unique characteristics that are captured as attributes. Just as real world objects interact with one another, so do entities in the E-R model, and they do this by way of relationships. Of all the alternatives to the relational model proposed since its introduction, the Entity-Relationship model has perhaps garnered the most support, albeit not in the manner its author had intended.
In large part, the failure of the E-R model to catch on as a serious data model derives from its informality: the very attribute that makes it so appealing in the first place. Because the E-R model is not derived from formal principles, it is art masquerading as science, greatly lacking in precision and predictability, both of which are the most appealing attributes of the relational model.
Of all the models proposed to replace the relational model in his lifetime, Codd himself found the E-R model to be the most objectionable. He writes, “The major problem in the entity-relationship approach is that one person’s entity is another person’s relationship. There is no general and precisely defined distinction between these two concepts, even when discussion is limited to a particular part of a business that is to be modeled by means of a database. If there are 10 people in a room and each is asked for definitions of the terms “entity” and “relationship,” 20 different definitions are likely to be supplied for each term” (Codd, 1990, p. 477).
The lack of precision in the definition of relationships in the Chen model is also noted by Kent (2000) and underscored by the ongoing research effort to generalize the concept and to provide a formal definition almost 30 years after the E-R model was originally described (see, for example, Dey, Storey, and Barron, 1999 and Wand, Storey, and Weber, 1999).