A Comparison of Three Systems Modeling Methodologies - High-Performance Web Databases: Design, Development, and Deployment - page 193

Exhibit 13-2. Entity-relationship model showing a unary relationship.
languages, the languages spoken by a person can be multivalued. An
address is an example of a composite attribute, because it is composed of
several more basic attributes. A derived attribute is one whose value can
be calculated through the use of other attributes.
Entities that share the same attributes are grouped into entity types and
are distinguished by the values of their attributes. For example, a com-
pany's employees would be grouped into an employee entity type.
The other basic component of an ER diagram is the relationship. A rela-
tionship is the association between entity types. It is represented by a dia-
mond with lines connecting it to each related entity. The name of the rela-
tionship is written in the middle of the diamond (see Exhibit 1).
Relationships have three important properties: degree, cardinality, and
participation. The degree of a relationship is based on the number of enti-
ties that are involved in the association. When an association occurs with
only one entity it is called unary or recursive. Exhibit 2 illustrates such a
relationship. The most common type of relationship is the binary type.
This relationship has two entities involved in the association. All of the
relationship in Exhibits 3 and 4 are binary relationships.