Chapter 4 Building Conceptual Data Models

Defining associations in a CDM

An association is a connection between entities. In the Merise modeling methodology an association is used to connect several entities that each represents clearly defined objects, but are linked by an event, which may not be so clearly represented by another entity.

Each instance of an association corresponds to an instance of each entity linked to the association.

Example

Three entities VIDEOK7, CLIENT, and STORE contain video cassette, client, and store information. They are linked by an association which represents a video cassette rental (K7RENTAL). The K7RENTAL association also contains the attributes DATE and STAFF_ID, which give the date of the rental, and the identity of the staff member who rented out the video cassette.

When you generate a PDM, K7RENTED is generated as a table with five columns, STORE_ID,K7_ID, CLIENT_ID, DATE, and STAFF_ID.

In PowerDesigner you can use associations exclusively in your CDM, or use both associations and relationships in the same model.

Association links

An association is connected to an entity by an association link. An Association Link symbolizes the role and the cardinality between an association and an entity.