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From:doug Date:September 20 2005 5:30pm
Subject:Re: Yet another null question
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First - thank you. I want to again thank this list, which in general is the most
tolerant to people new to database use and theory in general, and me in
particular. I mostly got your point from an earlier answer. Hopeful your example
quashes my ignorance, relative to NULL at least.

I think I understand the manual examples with the help of the answers I got. I
was not getting it on my own.

On Tue, 20 Sep 2005, Joerg Bruehe wrote:

> Hi!
>
>
> Just some explicit addition:
>
> doug@stripped wrote:
> > [[...]]
> >
> > So in the following query:
> >
> >  select * from new_payments where closed<>1;
> >
> > it is desired that null=1. DeMorgan's law takes a vacation here.
>
> You use two-valued logic here, where statements are either "true" or
> "false". (DeMorgan's law applies to two-valued logic only.)
>
> When NULL values are not excluded, SQL uses a three-valued logic, where
> a stament may also be "unknown". Comparing NULL to any value (including
> a comparison of NULL and NULL) always results in "unknown".
>
> This also the reason that the SQL syntax does not allow
>     ... WHERE value = NULL
> but requires that you write
>     ... WHERE value IS NULL
> It has also been said that NULL is no value but a state - maybe that
> helps in understanding.
>
>
> HTH,
> Jörg
>
> --
> Joerg Bruehe, Senior Production Engineer
> MySQL AB, www.mysql.com
>

_____
Douglas Denault
http://www.safeport.com
doug@stripped
Voice: 301-469-8766
  Fax: 301-469-0601
Thread
Yet another null questiondoug19 Sep
  • Re: Yet another null questionRoger Baklund19 Sep
    • Re: Yet another null questiondoug19 Sep
      • Re: Yet another null questionJasper Bryant-Greene19 Sep
      • Re: Yet another null questionJoerg Bruehe20 Sep
        • Re: Yet another null questiondoug20 Sep
    • Re: Yet another null questiondoug19 Sep
  • Re: Yet another null questionMartijn Tonies19 Sep
    • Re: Yet another null questiondoug19 Sep