Entity-relationship diagrams

This outline is adapted from Chen, P., 1991, The entity-relationship approach to logical database design and Jackson, G., 1988, Relational database design with micro-computer applications. (A slightly different format is described in Kroenke, D.M., 2000, Database processing: fundamentals, design & implementation, 7th ed.)

An entity class is represented by a rectangle, with the entity class name in the rectangle and the entity key name below.

A relationship is represented by a diamond connected by a line to each of the entity classes involved. A solid circle is placed at the end of each line. The maximum cardinality of the relationship is indicated by a number ("1") or letter ("n", "m") next to each line.

If participation of the instances of an entity class in the relationship is obligatory, the solid circle is enclosed in a small box. (The example shows that every course must be taught by a faculty member.)

If one entity class includes another, an arrow is drawn from its rectangle to the other entity class's rectangle.

An attribute is represented by an arrow to a circle representing a value type. If an attribute may be multivalued, the notation "1":"n" is used with the arrow. The name of the value type appears in the circle. The name of the attribute may appear next to the arrow, but this is not necessary if the value type name is the same.

A relationship may have an attribute; for example, the relationship of borrowing between a patron and a book may have attributes such as Date borrowed and Date due.

More than one attribute may have the same value type: Date borrowed and Date due both belong to the value type Date.


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Last updated July 5, 2001.
This page maintained by Prof. Tim Craven
E-mail (text/plain only): craven@uwo.ca
Faculty of Information and Media Studies
University of Western Ontario,
London, Ontario
Canada, N6A 5B7