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From: -CELKO- <jcelko212@earthlink.net>
Newsgroups: comp.databases.theory
Subject: Re: Examples of SQL anomalies?
Date: Mon, 30 Jun 2008 08:52:03 -0700 (PDT)
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>> If I want the total amount invoiced in a time period and there are no invoices, the total amount invoiced was zero. <<

You have to coalesce the NULL to a zero or drop it as not in the set.
Remember this one from freshman logic when the book got to the Greek
paradoxes?

1) A cat has one more tail than no cat.
2) No cat has 12 tails.
Ergo; a cat has 13 tails.

This is based on confusing the **quantity** zero with the absence of
an entity to have a quantity.  Get a copy of "Zero: The Biography of a
Dangerous Idea" by Charles Seife or "The Nothing that Is: A Natural
History of Zero" by Robert Kaplan and Ellen Kaplan.  There is also a
kid's book, A Place for Zero: A Math Adventure" by Angeline Sparagna
Lopresti and Phyllis Hornung, but it does not get into set theory.