a key that has an external, real-world meaning (such as a person's name, a book's isbn, or a car's serial number) is sometimes called a "natural" key. if no natural key is suitable (think of the many people named "brown"), an arbitrary or surrogate key can be assigned (such as by giving employees id numbers). in practice, most databases have both generated and natural keys, because generated keys can be used internally to create links between rows that cannot break, while natural keys can be used, less reliably, for searches and for integration with other databases. (for example, records in two independently developed databases could be matched up by social security number, except when the social security numbers are incorrect, missing, or have changed.)