Chapter 4 Building Conceptual Data Models

Defining a dependent association in a CDM

In a dependent association, one entity is partially identified by another. Each entity must have an identifier. In some cases, however, the attributes of an entity are not sufficient to identify an occurrence of the entity. For these entities, their identifiers incorporate the identifier of another entity with which they have a dependent association.

Example

An entity named Task has two entity attributes, TASK NAME and TASK COST. A task may be performed in many different projects and the task cost will vary with each project. To identify each occurrence of TASK COST the unique Task entity identifier is the compound of its Task name entity attribute and the Project number identifier from the Project entity.

When you generate a PDM, the TASK table contains the PROJECT NUMBER column as a foreign key, which is also a primary key column. The primary key therefore consists of both PROJECT NUMBER and TASK NAME columns.

To define a dependent association:

1. Double-click an association link symbol to display the association link property sheet.
2. Select the Identifier check box.
3. Click OK.
   
   The cardinality of the association link is enclosed in parenthesis to indicate that the association link is an identifier.

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