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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
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