The relationship shows how the data are related to one another within the organization or unit in question. A relationship is depicted in the ERD as a diamond shape. There are two key concepts to understanding relationships between or among entities. The first is the degree of the relationship. The degree of relationship refers to the number of entity types that are a part of the relationship.
Most entity relationships are binary.
The second key concept is the cardinality of a relationship. Cardinality of a relationship is the number of instances of entity B that can or must be associated with each instance of -entity A. Each relationship has a minimum and a maximum cardinality. The minimum cardinality can be zero, one or many. The maximum cardinality can also be zero, one or many. Consider the following example.