Chapter 4 Building Conceptual Data Models

Defining an entity attribute as mandatory or optional

If an entity attribute is mandatory, every entity occurrence must have a value for that entity attribute. If an entity attribute is optional, entity occurrences do not have to have a value for that entity attribute.

By default all identifiers are mandatory.

To define an entity attribute as mandatory or optional:

1. Double-click an entity in the diagram to display the entity property sheet.
2. Click the Attributes tab to display the corresponding page.
   
   It lists entity attributes associated with the entity.
3. Click the Entity attribute to define.
   
   An arrow appears at the beginning of the line.
4. Click the Properties tool.
   
   or
   
   Double-click the arrow at the beginning of the line.
   
   The entity attribute property sheet opens to the General page.
5. Select the Mandatory check box at the bottom of the dialog box to make the entity attribute mandatory.
   
   or
   
   Clear the Mandatory check box to make the entity attribute optional.
   
   You cannot clear the Mandatory check box for an identifier because it must always be mandatory.
6. Click OK in each of the dialog boxes.

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