5.1 Degree

The degree simply represents the number of entity types a relationship is associated with. In practice, most relationships have a degree of two. For recursive relationships, the degree is one (references to itself). Relationships can have higher degrees of three, four and up, virtually unlimited. The following image shows relationships with the most frequently occurring degrees:

Degrees (Chen Notation)
Degrees (Chen Notation)

The displayed relationships are often called unary, binary, or ternary relationships. Although degrees of four and up are possible, they aren't specifically named. The practical maximum for the degree is probably three or four. Degrees beyond are hard to understand and rather rare.

The degree is undoubtedly the highest-order relationship property. There is usually a pretty straightforward way to translate degrees higher than two to logical or physical models. However, there is a difference in how the degree is treated when translating referential and hierarchical relationships, explained in the next section, to the logical and/or physical level.

Last updated: 2010-10-13