Mr. Scott wrote:
....
I suspect there is no problem that can't be instantaneously solved with
a new kind of null, but only momentarily.

"paul c" <toledobythesea (AT) oohay (DOT) ac> wrote

I was being facetious.

Mr. Scott wrote:
Quite right too, most of db theory is that way.

paul c <toledobythesea (AT) oohay (DOT) ac> wrote:

So it is good that there are so many different nulls.
</sarcasm>

Sincerely,

Gene Wirchenko

Computerese Irregular Verb Conjugation:
I have preferences.
You have biases.
He/She has prejudices.

"Mr. Scott" <do_not_reply (AT) noone (DOT) com> wrote

The difference between these three kinds of nulls (and the other seventeen
kinds of nulls) is not in what the null says about the real world. It''s
all about the state of mind of the writer.
Reread what you've written here... it's all about the narrator knows or
doesn't know or doesn't have a clue about.

So what does a null say about the real world? Nothing.

I'm gl;ad you brought up the OWA/CWA question with regard to the middle
initial table. The question about what a missing row means in the Middle
Initial table is precisely the same question as to what a missing value
means in the middle initial column in the plain old one table design. And
you could come up with a OWA convention or a CWA convention with regard to
this table (or any table), but that is a convention about the semantics of
the data. More precisely, it's a convention about the semantics of the data
that isn't there.

That's beyond the scope of the point I was trying to make. My point was
just that if you think of a table containing nulls as a materialized outer
join, it can help you to avoid getting all balled up in SQL's three valued
logic and like that.

Sorry for the delay in responding. It's been a busy week.

"Walter Mitty" <wamitty (AT) verizon (DOT) net> wrote

I think it's about what should be considered information and what shouldn't.
The closed world assumption effectively denies every valid atomic formula
that can't be derived from what has explicitly been stated, so it would be
redundant to assert negative information. An inapplicable null states that
there is not supposed to be a value in a specific role. An
I-don't-have-a-clue null essentially states that it doesn't matter whether
there is a value in a specific role. In neither instance does the fact
being asserted depend on there being a value in that specific role, but
where there is an applicable null, the fact being asserted does depend on
there being a value, even if it isn't clear which one. This is why I think
that only applicable nulls convey information that can't otherwise be
derived from what has explicitly been stated.

An applicable null indicates that there is supposed to be a value in a
specific role drawn from a particular domain. That's quite a bit more than
nothing.

The point is that the semantics of the data is unclear when there are
I-don't-have-a-clue nulls or when a table has the open world interpretation.
It is not clear whether there is supposed to be a value.

"Mr. Scott" <do_not_reply (AT) noone (DOT) com> wrote

No problem.

You are confusing what the null indicates with what the cell containing the
null indicates.

The null indicates that there is no data in the cell. That is all. It is
the cell that might
indicate by its presence that there is supposed to be data there.