Figure 1. Binary relationship set
The number of connections to object classes in a relationship set is called the arity of the relationship set. If there are two connections, the relationship set is binary. If there are three, it is ternary. For four object classes the relationship set is quaternary. Relationship sets with five or more are 5-ary, 6-ary, and so on. It is common to refer to relationship sets of arity three or more as n-ary relationship sets.
Figure 2. Ternary relationship set
In the shorthand notation for binary relationship set names, we omit the diamond and the object class name, and add a directional arrow. In the shorthand notation, we thus represent the relationship set in Figure 1 as shown in Figure 3. We provide no shorthand notation for n-ary relationship sets.
Figure 3. Relationship set with abbreviated name
The analysts can express several different types of constraints on relationship sets.
Certain types of relationship sets appear so often in ORM diagrams that we have chosen special symbols to represent them. These include:
Here is a short quiz so you can test your understanding of relationship sets.
Go to the OSA Tutorial ORM
Go to Relationships Generalization/Specialization
Last updated 3 Nov 1994. by Mingkang Xu (firstname.lastname@example.org), Lei Cao(email@example.com)