On fös, 2008-11-28 at 15:22 +0100, Lutz Steinborn wrote:
> Hello Paul,
>
> thanks for the quick answer.
>
> > NULL values?
> Jepp, thats it.
> I've supposed this but can't believe it. So NULL is something out of this
> dimension :-)
Yes, that is one way of putting it.
A more useful way to look at it is to say that NULL represents an
UNKNOWN value.
rows with NULL bar value will not be counted by:
> > select count(*) from foo where bar = 'Mr Spock';
because you cannot know whether the UNKNOWN value is 'Mr Spock'.
these rows will neither be counted by:
> > select count(*) from foo where NOT bar = 'Mr Spock';
because you also cannot know that the UNKNOWN value is NOT 'Mr Spock'.
Does that make it any clearer?
gnari