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On Tue, Oct 11, 2011 at 3:54 PM, Raul A. Jager W. <raul@xxxxxxxxxx> wrote:
In language theory, a set containing only the "empty string" is
different from an empty set.

True enough, but an empty set isn't necessarily the same thing as
null, either. Conceptually, null (in the classic database sense)
means "missing or unknown value". When this is enforced, nulls do not
equal anything else, including other nulls. It effectively makes
logic three-valued (true/false/unknown) instead of binary
(true/false).

Empty sets (in the usual mathematical or computer science sense, and
most programming languages) are simply collections with no elements.
There is nothing "missing or unknown" about empty sets; two empty sets
compare equal to each other.

John

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