Newbie: Truth values (three-valued logic)

[Olaf Delgado]

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

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