Software Engineering-Entity/Relationship Diagrams


The object/relationship pair is the cornerstone of the data model. These pairs can be represented graphically using the entity/relationship diagram. The ERD was originally proposed by Peter Chen for the design of relational database systems and has been extended by others. A set of primary components are identified for the ERD: data objects, attributes, relationships, and various type indicators. The primary purpose of the ERD is to represent data objects and their relationships.

Data objects are represented by a labeled rectangle. Relationships are indicated with a labeled line connecting objects. In some variations of the ERD, the connecting line contains a diamond that is labeled with the relationship. Connections between data objects and relationships are established using a variety of special symbols that indicate cardinality and modality .

The relationship between the data objects car and manufacturer would be represented as shown in fogure. One manufacturer builds one or many cars. Given the context implied by the ERD, the specification of the data object car  would be radically different from the earlier specification . By examining the symbols at the end of the connection line between objects, it can be seen that the modality of both occurrences is mandatory (the vertical lines).

Expanding the model, we represent a grossly oversimplified ERD of the distribution element of the automobile business. New data objects, shipper and dealership, are introduced. In addition, new relationships—transports, contracts, licenses, and stocks—indicate how the data objects shown in the figure associate with one another. Tables for each of the data objects contained in the ERD would have to be developed according to the rules.
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