Each entity must have a unique identifier. An identifier is an attribute of an entity that meets the following rules:
- It is unique across all instances of the entity.
- It has a non-NULL value for each instance of the entity for the lifetime of the instance.
- It has a value that never changes for the lifetime of the instance.
The unique identifier is important because it designates which instance of an entity is being dealt with. Identifier selection is critical because it is also used to model relationships. If an identifier for an entity doesn't meet one of the above rules, it could affect the whole data model.
Designers often invent an identifying attribute that has no other meaning except to serve as an identifying attribute. Because this attribute is invented and is completely unrelated to the entity, the designer has full control over it and a guarantee that it meets the rules of unique identifiers.
A unique identifier is diagrammed as an underlined or italicized attribute.
The entities with unique identifiers