Re: So what's null then if it's not nothing?

In article <1134398244.343117.214050_at_g44g2000cwa.googlegroups.com>, boston103_at_hotmail.com says...
>
> Abandoning the fancy intension/extension language, one can also say.
> OK, in the programming language context we do not care about
> expressions but rather the values the expressions evaluate to. So, in
> this context, '==' can always be interpreted as the equality
> sign/predicate.

Yes! Being a programming language kind of guy, this is what I have meant all along.

> In short, I suggest that we'd better talk in terms of equality of
> values in the context of algebraic expressions (like x+3 == x-1+4)
> forgetting about the idea of expression equivalence since, in
> practical tems at least, that's how people usually think about this
> sort of stuff.

Agreed.

> > > > I still don't understand what the 'lethal' problem is. Can you give an
> > > > example?
> > >
> > > Evaluating w=w to w where = denotes equality.
> >
> > That may be the *cause* of a problem. What is the *problem*?
>
> Not being able to say whether two truth table for arbitrary expressions
> are in fact the same. Did not I say that, like, a dozen times ?

Wouldn't it be easy to fudge it using some IS NULL construct? And when do you need to do this anyway in a database context? I am looking for an scenario as concrete as my 3VL capital knowledge example; that's why I'm not satisfied with what you've said a dozen times before.

> > And for what it's worth: The SQL-99 standard seems to have interpreted
> > Codd the way I do. But I'm not sure if that weakens or strengthens my
> > position. :)
>
> Codd, at least, never called a contraption with three wheels a
> bicycle.

Well, it's just a name. Just a language design thing, no logical flaw.

> They also define a number of predicates, like '=', '>' over
> their 'Boolean' domain not even realizing that '=' is in fact Kleene's
> biconditional/equivalence, but they probably never heard about the
> fellow so it's hard to blame them.

I don't see why '<' is something to get worked up about. Why is it bad to define some (arbitrary) order for truth values? As for defining a strange '=', they do so for the other domains (and I think Codd does it too), and Codd's article doesn't mention Kleene either. It seems strange if Codd didn't bother to correct the SQL committee if it got things so very wrong.

-- 
Jon