Re: WWW/Internet 2009: 2nd CFP until 21 September x
Date: Wed, 19 Aug 2009 03:46:42 GMT
"Mr. Scott" <do_not_reply_at_noone.com> wrote in message
>> solution to be a materialized outer join gives a way of conceptualizing
>> the two tables, where no NULLS are necessary or permitted. It also cuts
>> back on the proliferation of tables. I'm all for decomposing tables when
>> it will really do some good, but there's no need to make a religious
>> ritual of it.
> I'm now wondering if there are really three kinds of null. There is the
> applicable null, which when submitted in an update indicates 'I know there
> is supposed to be a value here but I don't know what it is,' there is the
> inapplicable null, which when submitted in an update indicates 'I know
> there is not supposed to be a value here,' and now there is the
> I-don't-have-a-clue null, which when submitted in an update indicates 'I
> don't know if there is supposed to be a value here.' In order to simulate
> the I-don't-have-a-clue null that can be in your Employee table, your
> Employee_Middle_Initial table has to have the open world interpretation,
> so that whenever a row is missing it indicates that either there should be
> a value here but it is at present unknown or that there shouldn't be a
> value here.
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. Received on Tue Aug 18 2009 - 22:46:42 CDT