Entity identifiers enable their relationships to be modeled.

A relationship describes a binary association between two entities. A relationship may also exist between an entity and itself. Such a relationship is a recursive relationship. Each entity within a relationship describes and is described by the other.

Each side of the relationship has two components: a name and a degree. Each side of the relationship has a name that describes the relationship. Degree, also referred to as cardinality, states how many instances of the describing entity must describe one instance of the described entity.

Degree is expressed using two different values: "one and only one" (1) and "one or many" (M).

Sometimes it is helpful to express a relationship verbally. One way of doing so is to plug the various components of a direction of the relationship into the formula:

Entity_A has [one and only one | one or many] Entity_B

In data models, a relationship can be shown by drawing a line between the two entities. Degree is expressed with a straight line for "one and only one" relationships or crows feet for "one or many" relationships.

A relationship between two entities