Surrogate keys

If "numeric pseudo-keys" and "surrogate keys" are at least overlapping concepts, then
the following deserves mention:

Surrogate keys are used in cases where key discipline is beyond the control of the enterprise that owns the database. If you can't guarantee that every new entry will have one and only one key value, and that every key value will be used at most once, better go with a surrogate key.

An example: every now and then the Social Security Administration screws up, and assigns the same Social Security number to more than one person. Hopefully, that doesn't happen as often as it used to, and it was always a very rare occurrence. But do you want your system to fail when that happens?

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Regards,
    David Cressey
    www.dcressey.com