An ''auto-generated key'' is a key for a RelationalDatabase table that rather than reflecting some element or attribute from the problem domain or the data/relationships to be stored in the database; is generated automatically somehow--and doesn't have any meaning, other than being a unique MagicCookie which can unequivocally identify a particular record within a database.  

Sometimes, this is done because a particular record will be in one-to-one correspondence with a particular external entity, and said external entity has no particular attribute which is unique.  A customer ID number is one example; there is no globally accepted unique ID for persons that can be used.  Social security numbers come close in the US; though many object to their use for this purpose.  Names cannot be, in general, used for this purpose--lots of folks around with the same name.  (But don't tell that to the ScreenActorsGuild, the union representing waiter^H^H^H^H^H^Hactors in Hollywood.  The SAG requires that actors' screen names be unique; were a fresh young actor whose name were "Brad Pitt" to seek work in Hollywood; he would have to find a different ''nom de cinema'' to use, as "Brad Pitt" is already taken.  "Bradley Pitt", "Brad Q. Pitt", or any unused pseudonym would be acceptable.)

Other times, AutoGeneratedKey'''''s to give a particular record an ObjectIdentity, even when said record isn't in one-to-one correspondence with any particular real-world entity.  When a new record-cum-object is created, it is assigned a new AutoGeneratedKey.  As the record-cum-object is mutated, the other attributes of the key may change, but the auto-gen key stays the same.  When the record-cum-object is deleted from the database, the key is thrown away.  (Sometimes, such keys are recycled; but that is often considered a bad idea).

AutoGeneratedKey''''''s are, depending on your point of view, either a beneficial feature for database products or an ugly hack, allowing one to simulate non-relational data models in a RelationalDatabase.  

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See also: AutoKeysVersusDomainKeys [editor: merge in order?]
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CategoryRelationalDatabase