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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 15:28:17 -0800
Message-ID: <1133393297.013854.12360@g47g2000cwa.googlegroups.com>

Hugo Kornelis wrote:
> On 29 Nov 2005 07:11:29 -0800, vc wrote:
>
> (snip)
> > The 3vl unknown is mapped to false
>
> Hi vc,
>
> The 3vl unknown isn't mapped to anything.
>
> ANSI standard says:
> - For SELECT: return rows if the condition in the WHERE clause evaluates
> to true; omit rows if it evaluates to either false or unknown.
> - For CONSTRAINT: accept rows if the constraint's condition evaluates to
> true or unknown; reject rows if it evaluates to false.
>
> No mappping involved.

I attempted to simplify the UNKNOWN handling explanation, but apparently the standard exposition is better than mine. There is no mapping in the standard of course.

>
> >, therefore, null is
> >*not* a duplicate.
>
> You can't say that null is not a duplicate - it is unknown if it's a
> duplicate or not.

You took my words out of the context:

o null is not considered a duplicate in the context of the unique constraint enforcement. At the same time:

o two NULLs are not considered distinct
o the result of comparison to NULL is UNKNOWN

>
> Best, Hugo
> --
>
> (Remove _NO_ and _SPAM_ to get my e-mail address)
Received on Wed Nov 30 2005 - 17:28:17 CST

Original text of this message

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