Re: NULLS and User Input WAS Re: multimaster - Mailing list pgsql-general

On 6/4/07, Andrew Sullivan <ajs@crankycanuck.ca> wrote:
> On Mon, Jun 04, 2007 at 12:37:42AM +0200, PFC wrote:
> > NULL usually means "unknown" or "not applicable"
>
> Aaaargh!  No, it doesn't.  It means NULL.  Nothing else.
>
> If it meant unknown or not applicable or anything else, then
>
>         SELECT * FROM nulltbl a, othernulltbl b
>                 WHERE a.nullcol = b.nullcol
>
> would return rows where a.nullcol contained NULL and b.nullcol
> contained NULL.  But it doesn't, because !(NULL = NULL).

I don't disagree with the principle, but that's a specious argument.
Who says (unknown = unknown) should equal true?

Alexander.