Re: 3 value logic. Why is SQL so special?

Bob Badour wrote:
> peter koch wrote:
>
> > Bob Badour wrote:
> >
> >>Chris Lim wrote:
> >>
> >>
> >>>Roy Hann wrote:
> >
> > [snip]
> >
> >
> >>Then why have I had to spend so much time in my career explaining to
> >>reasonably intelligent people why their queries returned the wrong answer?
> >
> > [snip]
> >
> >
> >>I must insist you back up that statement quantitatively and
> >>qualitatively. It is far easier to deal with two names than with
> >>surprisingly inconsistent semantics for the same reason it is far easier
> >>to deal with a compile-time error than a run-time error.
> >
> > [snip]
> >
> >> But a
> >>
> >>>database without NULLs? It might be theorectically correct, but it
> >>>would be a nightmare to write queries against.
> >>
> >>I disagree. My personal experience dealing with scores of intelligent
> >>database users suggests that NULL is the nightmare.
> >
> > All these arguments against NULL are only valid against some specific
> > implementations of that concept - here SQL in its various dialects.
> > They are not arguments against the concept of having null-values (e.g.
> > to represent unknown values) in some database system. And this is
> > comp.databases.theory after all.
>
> I have found the arguments against 3VL and NVL for N > 2 compelling. For
> example, if DEE and DUM are canonical relations for true and false, what
> are the canonical relations for the other logical values?

Other logical values? I believe I miss your point here. If we can agree that there is a basic boolean type which can take the values true and false (this is all we need in order to create other, more complex types), there could be a nullable type named e.g. nullable<boolean> consisting of a pair of booleans.
If you prefer to see this as tables/relations this is alright with me - but why on earth the names DEE resp. DUM - and how do you describe anything at all if the logical types are not primitive?

>
> We currently have no theory that I am aware of for dealing with missing
> information. At this time, I cannot even imagine what such a theory
> would look like because science and the scientific method have such
> strong roots in empiricism. Hopefully someone much smarter than me will
> come up with a sound one, though.

That I do not understand either. Somewhere else in this thread you gave an example where missing information was involved. Do you mean to say that that example was meaningless?

>
> The whole point of NULL is to have something other than a value to
> represent missing information. To use a value to represent unknown, what
> we really need are extensible data type systems that as much as possible
> enforce consistency. For example, if SUM is an aggregate of +, an
> extensible data type system should enforce consistent semantics +
> semantics for SUM.
I would say that "+" was defined with respect to a type (or a set of types) and that SUM (in its least generic form) must be defined in terms of +. Of course since for a given type T, nullable<T> extends T and depending on both nullable and T the meaning of SUM could be given different interpretations (as could SUM also for a given T)

/Peter Received on Tue Sep 19 2006 - 15:28:54 CEST