We can model much of a system with objects , relationships, object classes, and relationship sets. To describe a system more satisfactorily, however, we often wish to state additional properties of object classes and relationship sets by imposing constraints.

For instance, we might want our model to allow only one Age object for each Person object, or at most one object in the President object class when modeling a company.

In an ORM, a constraint restricts the membership of one or more object classes or relationship sets.

The Object-Relationship Model allows analysts to express several different types of constraints:

Special relationship sets including generalization, specialization, aggregation, and association impose restrictions directly on relationship sets and indirectly on object classes.

Here is a short quiz so you can test your understanding of constraints.

Go to the OSA Tutorial ORM

Go to Association Participation Constraints

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