From: Martin B. Jespersen
Date: March 30 1999 2:49pm
Subject: Re: x NOT IN (y)  <=>  x != y
List-Archive: http://lists.mysql.com/mysql/1190
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Corin Hartland-Swann wrote:
> 
> Hi there,
> 
> While trying to get my head around NULL values, I stumbled on the
> following:
> 
>    CREATE TABLE foo (bar int);
> 
>    INSERT INTO foo VALUES (NULL),(1),(2);
> 
>    SELECT * FROM foo WHERE bar NOT IN (1);
> 
> returns NULL and 2
> 
>    SELECT * FROM foo WHERE bar != 1;
> 
> returns 2
> 
> NULL is supposed to mean "unknown value", so surely there is no way of
> knowing whether the row where bar = NULL is 1 or not 1.
> 
> I'd always worked under the assumption that
> 
>    x NOT IN (y)        <=>       x != y
> 
> Can anyone explain this?
> 
> Thanks in advance,
> 
> Corin
> 

Hmm aren't you supposed to use <> and not != in SQL?

Anyways, One explanation might be that you can't use logical operators on NULL values.
That is the reason why you don't do = NULL but IS NULL.

-- 
Martin B. Jespersen
Technical Project Leader
Icon Medialab A/S (http://www.IconMedialab.dk)

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