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

From: vc <boston103_at_hotmail.com>
Date: 30 Nov 2005 17:23:17 -0800
Message-ID: <1133400197.315562.163950@g14g2000cwa.googlegroups.com>


I tried to respond to this twice, and the message(s) got lost. Briefly:

Jon Heggland wrote:
> In article <1133286593.699982.59590_at_g43g2000cwa.googlegroups.com>,
> boston103_at_hotmail.com says...
> >
> > Jon Heggland wrote:
> > > By the exact same argument, duplicate NULLs should *not* be removed,
> > > because they are not equal. Choosing one over the other is an arbitrary
> > > decision; an artifact of how you formulate the SELECT DISTINCT
> > > operation.
> >
> > No, the argument is not exactly the same. We have stipulated two
> > independent rules of the game:
> >
> > a. two nulls are not distinct; b. two nulls are not equal
>
> a. Projection removes all but one of tuples that are not distinct.
> b. Projection retains tuples so that all are not equal to any other.
>

You cannot use (b) because the equality is unknown for nulls. (a) is you only choice.

> Pick either one, but I can't see how you can claim that one is more
> correct than the other, except for the reason that Codd said so.
>
> > In the 3VL, neither implies the other.

Sorry, I did not notice 'not' in the second part. It should have been: " 'two nulls are not distinct' does not imply 'two nulls are equal'.

I think we've covered the rest elsewhere. If not, please let me know. Received on Wed Nov 30 2005 - 19:23:17 CST

Original text of this message

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