On Thu, Jun 08, 2006 at 06:09:21PM -0700, Trent Shipley wrote:
> On Thursday 2006-06-08 15:14, David Fetter wrote:
> > On Thu, Jun 08, 2006 at 05:21:07AM -0700, dananrg@yahoo.com wrote:
>
> > on bag theory[1] and 3-value logic[2]. Until they come up with a
> > testable system, or Hell freezes over, whichever comes first,
> > Pascal's book will make a good companion on your shelf to books on
> > Phlogiston[3] theory, or a decent doorstop, whichever you prefer.
>
> I have encountered at least two commercial database products that
> declared every column "NOT NULL". I have always assumed that this
> was defensive, preventing stupid programmer mistakes.
It's not that simple. If there are no NULLs allowed anywhere, that
means that you can't even have them as the output of a SELECT
statement, which means no OUTER JOINs. No repetitions means none
anywhere, which means that they can't be the output of a query either,
and makes it complicated at best to do aggregates. The whole thing is
just ridiculous on its face.
> I recall reading somewhere that Codd proposed multiple flavors of
> nullity. Are there theoretical proposals for databases with logical
> systems having more than three values?
Codd proposed two different NULLs, if I recall right, and some people
have come up with tens of different meanings that NULL might have,
which leads to some major silliness wherein your "truth table" is a
grid that doesn't legibly fit on a piece of A4 paper.
Cheers,
D
--
David Fetter <david@fetter.org> http://fetter.org/
phone: +1 415 235 3778 AIM: dfetter666
Skype: davidfetter
Remember to vote!