Relationship Sets

Relationship Set

A relationship set is a set of relationships, all of which In an ORM diagram, a relationship set is represented by a diamond with lines connecting associated object classes. There is a line for each connection to an object class.

For instance:

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.

For example:

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.

Special 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

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Last updated 3 Nov 1994.
by Mingkang Xu (, Lei Cao(