A natural key is a type of primary key that is not controlled by your system.
Examples are all state identifications such
- the VIN number for your car
- the VAT number of your company
- the SSN (social security number) for a person
This type of key is rarely used in practice as primary key because:
- they may change (they are not controlled by your system)
- they may not be available (people are not required to reveal their SSN)