I wrote some notes on three-valued logic (true,false,unknown) at my website:
http://www.comptechnews.com/~reaster/dbdesign.html#three-valued-logic
If anyone finds errors with it, please let me know.
On Wednesday 13 December 2000 19:03, Stephan Szabo wrote:
> On Wed, 13 Dec 2000, Al Lewis wrote:
> > I am migrating to postgress from msql and am encountering numerous
> > problems in the differences in NULL usage and comparison.
> >
> > 1. Why are 2 fields not equal if they are both NULL?
>
> Because that's what the SQL spec says. If either value is NULL
> the result is unknown (because NULL is not a value really,
> is this unknown value equal to some other unknown value...).
>
> > 2. Is there a way to easily modify my sql select statements to account
> > for either column being NULL and having them return true if they both are
> > NULL? select a.name, b.cost from a, b where a.type=b.type
>
> where a.type=b.type or (a.type is null and b.type is null)
> should do it.
>
> > I'd like to make this as easy as possible so I can put it into a
> > "translation" function. Currently I have a regsub that handles <> and
> > NULLs, since <> doesn't work on a NULL field.
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