Chapter 4 Building Conceptual Data Models

Defining data items in a CDM

A data item is an elementary piece of information in a model. It represents a fact or a definition in an information system, which may or may not have any eventual existence as a modeled object.

You can attach a data item to an entity. It then becomes an entity attribute of that entity. When a physical data model (PDM) is generated from a CDM, an entity attribute generates a column in a table.

You can also define a data item, but not attach it to an entity. It remains defined in the model and can be attached to an entity at any time.

Depending on its defined model options, a data item can be unique in the model, or unique only for a particular entity. In the latter case, a data item is equivalent to an entity attribute.

Example

In the information system for a publishing company, the last names for authors and customers are both important pieces of business information. The data item LAST NAME is created to represent this information. It is attached to the entities AUTHOR and CUSTOMER, and becomes entity attributes of those entities.

Another piece of information is the date of birth of each author. The data item BIRTH DATE is created in the CDM, but as there is no immediate need for this information in the model, it remains in the CDM, but is not attached to any entity.