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Entity-Relationship Model


In 1976 Dr. Peter Pin-Shan Chen’s paper "The Entity-Relationship Model: Toward a Unified View of Data" is published in ACM on Database Systems, Vol. 1, No. 1, March 1976 and receives much attention.

Entity–Relationship (ER) modeling is an important step in information system design and software engineering.

„Entities and relationships are a natural way to organize physical things as well as information … The ER concept is the basic fundamental principle for conceptual modeling. It has been with us since thousands of years ago and will be with us for many years to come.”

Dr. Peter Chen, SIGMOD Record, Vol. 33, No. 1, March 2004

The entity-relationship model can be used as a basis for unification of different views of data:  the network model, the relational model and the entity set model. In the entity-relationship model the semantic of data is much more apparent than in the other models. Also the handling of data ambiguity is much better than for example in the network model with its ambiguous arrows. In the entity-relationship model all kinds of mappings are handled uniformly. Not everything is treated as an entity such as in the entity set model and the entity-relationship model also uses n-ary relationships and not only binary relationships like the entity set model does.

Also a special diagrammatic technique is introduced as a tool for database design – the entity relationship diagram.

The entitiy-relationship model adopts the more natural view that the real world consists of entities and relationships.

In this paper Dr. Chen introduces the entity-relationship model using a framework of multilevel views of data. There can be identified four levels of views of data. The entity relationship model is mainly concerned with the first two levels. The first level deals with information concerning entities and relationships that exists in our minds. The second level deals with information structure concerning the organization of information in which entities and relationships are represented by data.

Information concerning entities and relationships (Level 1)


Figure 1. Levels of views of data.


In this paper Dr. Peter Chen defines an entity as a thing that can be distinctly identified. An example of an entity is a specific person, a company or an event. A relationship is an association among entities. Entities are classified into different entity sets. A relationship set is a mathematical relation among a number of entities, each taken from an entity set. The role of an entity is the function that it performs in the relationship.

Values are classified into different value sets. An attribute can be formally defined as a function that maps from an entity set or a relationship set into a value set or a Cartesian product of value sets.


Figure 2. Attributes and values defined on an entity set.


Information Structure (Level 2)

Entities, relationships and values, described at level 1, are conceptual objects in people’s minds. At level 2 there is the consideration how to represent entities and relationships. Also primary keys are introduced to identify different entities in an entity set.



Figure 3. Entity relation in table form with a primary key.

Information about entities in an entity set and relationships can be organized in table form. The whole table is an entity relation and every row is an entity tuple.

Entity-Relationship Diagram

The entity relationship diagram is a diagrammatic technique to exhibit entities and relationships.



Figure 4. An entity relationship diagram .

Each entity is represented by a rectangular box and each relationship is represented by a diamond-shaped box. Lines, that connect the rectangular boxes, represent that the relationship set is defined on the connected rectangular boxes. The diagram is able to distinguish between 1:n, m:n and 1:1 mappings.

„Certain things can not be expressed naturally in an object-oriented way. My point 25 years ago, and my point today, is that you must do things in a natural way.“

Dr. Peter Chen, SIGMOD Record, Vol. 33, No. 1, March 2004

More information

Dr. Peter Chen's homepage

Reference

Chen, P.P., "The Entity-Relationship Model: Toward a Unified View of Data," ACM Trans. on Database Systems, Vol.1, No.1, March 1976, pp. 9-36.

Chen, Peter P., Entity-Relationship Modelling: Historical Events, Future Trends, and Lessons Learned, Springer-Verlag New York, Inc, 2002, pp. 296-310.

Winslett, Marianne : Peter Chen Speaks Out on Paths to Fame, the Roots of ER Model in Human Language, the ER Model in Software Engineering, the Need for ER Databases and More, SIGMOD Record, Vol. 33, No. 1, March 2004