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There is a branch of mathematics that deals with "sets". Relational databases are based in sets and folow the rules of this branch of mathematics.
_______________________________________________________________________________________
Joe Pluta wrote:

From: Raul A. Jager W.

We realy are dealing with math here, and there are clear rules about
operations.  I will not argue if the rules are correct or if they make
sense.  Just as 2 + 2 = 4 wether you like it or not, the nulls have
their rules.

NULLs are not math in the sense that they follow some established rules
of mathematics.  They are simply a convention made up by people who
chose to agree on how they would work, and they may or may not work for
someone else.  The closest thing you will find is the concept of an
"undefined value" in a function (such as the result of one divide by
zero), but even that doesn't apply to an alpha field.

Nowadays programmers have a bad tendency to take whatever anybody tells
them as "rules" or "laws" when they are simply conventions.  As I
explained, the conventions for how NULLs work are not necessarily
correct for every business situation.


Nulls are an excelent way to deal with some bussiness rules, it is up
to
us, IT people to decide when to use nulls and when to use something
else.
SQL must implement the nulls in the way the matematicians defined they
must behave.  If you need some other behavior, don't use nulls.

As far as I know, there were no professional mathematicians involved in
the design of NULL.  Just a bunch of IT guys sitting around trying to
figure out what to do.  They may have taken a course in math at one
point and applied their knowledge to the question, but the idea that
NULL means "undefined" is simply a convention.  In fact, in modern
database theory there are at least three different meanings of NULL: the
attribute exists but is currently unknown, the attribute does not exist
or is inapplicable, or we don't know whether the attribute is
applicable.  (Example: Employer's Name.  Unknown: the person is
employed, but we don't know where.  Inapplicable: The person is not
employed.  Neither: We don't know whether they're employed or not.)

My problem with NULLs is not how they are implemented, but that they are
simply not flexible enough.  A numeric null should have the ability to
be treated as a zero or indeed as any number I choose.  The same with an
alpha field; I should be able to easily cause all NULL values to act how
*I* want them to act, not how somebody else thinks they should act.

Joe


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