Chapter 3 Building Conceptual Data Models

Defining a mandatory relationship

The property Mandatory indicates whether the relationship between entities is mandatory or optional. You define options from the point of view of both entities in the relationship.

Termination points

In the resulting relationship symbol, a bar represents a mandatory relationship. A circle represents an optional relationship.

Termination point	Existence	Cardinality	Description
	Mandatory	One	One and only one must exist
	Mandatory	Many	One or more must exist
	Optional	One	None or one may exist
	Optional	Many	None or more may exist

For more information on IDEF1X existence notation, see the CDM options section in the Conceptual Data Model Basics chapter.

To define a mandatory relationship:

1. Open the relationship property sheet and click the Cardinalities tab.
2. Select the Mandatory check box in one or both of the groupboxes corresponding to a relationship direction.
3. Click OK to return to the model.

Example

The Cardinalities page below indicates that the subcontract relationship is optional from customer to project, and mandatory from project to customer. Each project must have a customer, while each customer does not have to have a project.

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