Defining cardinality for relationships

Chapter 4 Building Conceptual Data Models

Defining cardinality for relationships

Cardinality indicates the number of instances (none, one, or many) of an entity in relation to another entity. You can select the following values for cardinality:

Cardinality	Symbol	Description
One-to-one	<1..1>	One instance of the first entity can correspond to only one instance of the second entity
One-to-many	<1..n>	One instance of the first entity can correspond to more than one instance of the second entity
Many-to-one	<n..1>	More than one instance of the first entity can correspond to the same one instance of the second entity
Many-to-many	<n..n>	More than one instance of the first entity can correspond to more than one instance of the second entity

The Cardinalities page contains a groupbox for each direction of the relationship. The title of the groupbox takes the following form:

'EntityA' to 'EntityB'

The following properties apply to each direction of the relationship:

Property	Description
Dominant role	In a One to One relationship, indicates to generate a reference in the PDM for this direction of the relationship only
Role name	Text that describes the relationship of EntityA to EntityB
Dependent	Indicates that each instance of the EntityA is identified by an instance of EntityB
Mandatory	Indicates that each instance of the EntityA requires an instance of the EntityB
Cardinality	Maximum and minimum number of instances of EntityA in relation to EntityB (if mandatory, at least 1). You can indicate the following cardinalities: 0..1 0..n 1..1 1..n

Termination points

In a diagram, termination points indicate cardinality at each end of a relationship. A termination with a single contact point denotes a cardinality of one. A termination with three contact points denotes a cardinality of many.

Termination point	Cardinality
	One
	Many

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

To define cardinality:

Double-click the relationship to display the relationship property sheet.

It displays the definition of the selected relationship.

Click the Cardinalities tab to display the corresponding page.

Select one of the cardinality radio buttons.

Click OK.

Example

The Cardinalities page below indicates that an employee must belong to one and only one division, while a division must include one or more employees.