Jamie Collins wrote:

You state that you "know that the ISBN is an excellent example of a
key" -- on what do you base this assertion?

In a nutshell, because Joe Celko tells me so g.

But Joe would also remind you that "a key" does not mean "a single field".
A key can be (and often is) made up of multiple fields. In fact a natural
key purist like Mr. Celko would likely have multiple column keys the vast
majority of the time because few entities have a single attribute that is
unique.

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