Entity Types and Key Attributes

Entities with the same basic attributes are grouped or typed into an entity type.
For example, the entity type EMPLOYEE and PROJECT.

An attribute of an entity type for which each entity must have a unique value is called
a key attribute of the entity type.
For example, SSN of EMPLOYEE.

A key attribute may be composite.
Vehicle Tag Number is a key of the CAR entity type with components (Number, State).

An entity type may have more than one key.
The CAR entity type may have two keys:
VehicleIdentificationNumber (popularly called VIN)
VehicleTagNumber (Number, State), license plate number.

Each key is underlined