Newbie: Truth values (three-valued logic)
Olaf Delgado
delgado at Mathematik.Uni-Bielefeld.DE
Fri Jun 18 11:55:39 EDT 1999
On Fri, 18 Jun 1999, Gordon McMillan wrote:
> Note that, if you want to retain DeMorgan's Laws, you don't have
> much choice. ANSI SQL defines a three valued logic which meets these
> conditions, and comes up with
> NOT maybe == maybe
> (well, NOT unknown == unknown), which makes sense as long as you
> rmember that NOT is an operator, and the result of a non-trivial
> operation on an unknown operand is probably unknown.
>
> Which is why most DBA's won't allow NULL values in any column that
> might be used in a WHERE clause - three valued logic is often
> non-obvious.
Well, NOT unknown == unknown seem perfectly obvious to me, but then I'm a
Mathematician and everyone knows we're strange guys.
Obviously yours,
Olaf
--
//// Olaf Delgado Friedrichs, Uni Bielefeld, Mathematik
Olaf Postfach 100131 (room: V4-109)
`=' D-33501 Bielefeld, Germany (phone: +49 521 106-4765)
http://www.mathematik.uni-bielefeld.de/~delgado
More information about the Python-list
mailing list