A Comparison of Three Systems Modeling Methodologies - High-Performance Web Databases: Design, Development, and Deployment - page 192

Exhibit 13-1. Diagram notation for the entity-relationship model.
An attribute is a property of an entity or a relationship; however, some
models do not permit an attribute to a relationship. It is depicted as an
ellipse that is attached to an entity. The name of the attribute is placed in
the middle of the ellipse (see Exhibit 1).
Attributes can have many different roles. They can identify or describe.
When an attribute identifies an entity it distinguishes that entity from the
other entity of the same type. An example of this is a person's social secu-
rity number. If it is assumed that all social security numbers are unique,
then this attribute can be considered to be a primary key for this entity. A
primary key uniquely identifies an entity. This attribute should not be null
(this property is called entity constraint) or be changed often. It is repre-
sented as an attribute with the name underlined.
Attributes can be atomic, multivalued, composite, or derived. Anatomic
attribute has a single value. The age of a person is an atomic attribute. A
multivalued attribute can have several values. If a person speaks many