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


Jon Heggland wrote:
> In article <1134052281.291970.61920@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
> boston103@xxxxxxxxxxx says...
> > > > What you'd like to store is say a (person, location) pair (or tuple,
> > > > does not matter). You'd want to deal with a domain of strings,
> > > > extended by two markers, UNKNOWN and MISSING to represent an address.
> > >
> > > What are you talking about now? This has nothing to do with my example.
> >
> > That has a whole lot to do with yours or similar examples.
>
> I never mentioned locations or addresses.
>
> > Instead of
> > storing precomputed truth values/judgements, one should arguably
> > rather store actual facts.
>
> I do. A person's opinion on whether Oslo is the capital of Honduras is a
> fact just like the person's last name is, if it is opinions we are
> interested in. I don't care what the capital actually *is*, at least not
> enough to put it in the database.
>
> > You suggest to store:
> >
> > (person, opinion, veracity).
>
> No, actually. I meant (person, opinion, ...) (if it were a binary
> relation we wouldn't need 'missing' NULLS), where the domain of
> 'opinion' is { TRUE, UNKNOWN, FALSE }.
>
> > I suggest to store (person, opinion) where opinion is a set {opinion1,
> > opinion2,..., UNKNOWN} and then derive veracity.
>
> Of course it can be handled differently. I just don't see why my initial
> design should be disallowed.

OK, that's a different discussion altogether [whether we need to store
logical values in the database rather than derive them from stored
facts, both for the 2VL and a 3VL]. I am not entirely convinced we
need for other reasons than performance, but let's skip it for now.

> > > > > Codd says null=null is UNKNOWN. Either he is here redefining the notion
> > > > > of equality,
> > > >
> > > > Yes, he does.
> > >
> > > I don't think so. What would be the point of that? Where is this kind of
> > > equality ever used in the RM? I think he is saying that null=null is
> > > UNKNOWN in the context of a restriction, like a WHERE-clause in SQL.
> >
> > Sorry, but saying null=null evaluates to UNKNOWN is already a
> > redefinition of the equality predicate.
>
> ... unless he by '=' means equivalence!

I don't know about a single example of him using '=' as equivalence in
the logical domain. Could you provide one ?

>
> > You do not need to take anybody's word for that. It's rather an
> > obvious statement. Take for example De Morgan laws or any other
> > logical expression transformation. Saying that two such expressions
> > are *logically equivalent* means (by definition) that they have the
> > *same* truth tables. Now, with your suggested approach to redefine
> > equality for truth values,
>
> I don't suggest to redefine equality for anything. I don't think Codd
> does either.

But he does, what else 'null = 7--> unknown' is if not a redefinition
?

>
> > one would never be able to determine if two
> > truth tables are in fact *the same* as some attempts at such comparison
> > would produce UNKNOWN should any row of such table contain UNKNOWN or
> > some such.
>
> Fair enough. And this doesn't mean that arithmetic has "ceased to
> exist" in SQL due to a redefinition of equality of numbers, because
> arithmetic doesn't depend on comparing truth tables?

I do not understand the bit about arithmetic.

>
> > > Fascinating. How does this make any difference? If you would like to
> > > enlighten me, please tell me what '=' signifies when you compare two
> > > logical expressions in a WHERE clause in SQL.
> >
> > I might be able to tell if say what you mean by "compare two logical
> > expressions in a WHERE clause in SQL".
>
> What's so unclear about it? The very same examples I have used several
> times earlier. "select * from Foo where (a < 16) = (b > 7)" if a and b
> are numbers; "select * from Bar where a = not b" if a and b are truth
> values.

I thought we'd covered that earlier. The most likely interpretation is
'=' is a biconditional.

>
> > >Or in a restriction in the
> > > RM, to be more technology independent.
> >
> > For example ?
>
> A relational algebra or calculus expression---but we have to use a
> textual language anyway, so it doesn't really matter.

Please provide an example of such expression.

> --
> Jon

.

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