Re: Null in subquery returns no records

If NULL is considered to an "un" value then at least a three-valued logic must be employed to handle the fact that a NULL value is not equal to a NULL value, right?

A null set as the value of a variable is a value equal to another null set value.

The relational model itself does not require a three-valued logic and you can find Date and others argue in favor of a two-valued logic instead. There are numerous databases the employ the much more intuitive two-valued logic but I know of no implementations of the relational model that do.

Did that make sense? Please let me know if there is anything illogical or not understandable in this explanation so I can correct it. Cheers! --dawn

"Dan" <guntermannxxx_at_verizon.com> wrote in message news:FGbVb.244$4e3.185_at_nwrddc02.gnilink.net...
> What is the difference between a null, an "unknown value," and a null set?
>
> Is a null set an empty set?
>
> If a null set means "a set with an unknown set of values", then what good
> does that do us from a logical perspective. Isn't that set as worthless
as
> an unknown value?
>
> What is returned from a null set if you don't use nulls?
>
> How does a null set help keep fidelity to two-value logic?
>
> Thanks,
>
> Dan
>
> "Dawn M. Wolthuis" <dwolt_at_tincat-group.com> wrote in message
> news:c02sjp$q4n$1_at_news.netins.net...
> > "Jonathan Leffler" <jleffler_at_earthlink.net> wrote in message
> > news:z%0Vb.17554$uM2.2847_at_newsread1.news.pas.earthlink.net...
> > > Mike MacSween wrote:
> > > > New testing rule for Mike - always test with null values.
> > >
> > > Alternative rule - always ensure the database stores no nulls!
> > >
> > > Not necessarily easy - but a good discipline.
> > >
> > <snip>
> > And perhaps the best option of all -- use one of the databases that
> employs
> > a two-valued logic, with NULLs treated as null sets -- much cleaner and
> > clearer. Examples are jBASE from www.jbase.com , Revelation from
> > www.revelation.com and the U2 databases (UniData and UniVerse) from
> > www.ibm.com (then do a search). You would not be alone as there are
over
> > 100,000 companies that use these products (could be a low estimate).
> >
> > I'm sure there are others as well that have avoided many of the SQL and
> > RDBMS complications. There is nothing holy nor scientifically more
> correct
> > about the RDBMS model than many other possibilities. If you have an
> > opportunity to free yourself of the notion that an RDBMS is required,
you
> > are likely to find a more productive environment elsewhere.
> >
> > Just thought I'd toss in my two cents even though most people who share
> this
> > opinion keep it quiet. --dawn
> >
> >
>
>
Received on Mon Feb 09 2004 - 05:42:05 CET