Bob Badour wrote:
> JOG wrote:
> > topmind wrote:
> >
> >>Nulls are a vendor-specific idea, not inherent to relational.
> >
> > While I agree with much of your post, nulls now appear to me to be
> > essential to the relational model (whatever their rights and wrongs).
> > If one fully normalizes a database then it is likely a join will
> > require the use of null to pad the resulting virtual relation. Just
> > because these relations are not persisted on disk, makes them no less
> > 'relations' in the sense of Codd's algebra. As such i am currently
> > struggling with how one can be against nulls (as per Date's perfectly
> > justified view) but pro-RM from a completely theoretical standpoint.
>
> With all due respect, is it possible you confound NULL with missing
> information?
An excellent point. The fact that SQL NULLs sometimes behave
like "an unknown value" and sometimes not makes the situation
all the worse.
> Obviously, missing information is a difficult problem no matter what
> data model one uses. We currently have no theory regarding missing
> information which means we have no theory to overcome the practical
> problem in any data model.
Well, I would propose that we understand cardinality-0 relations
pretty well, and I think that provides a lot of value as far as
a theoretical basis goes. (This doesn't help us when using SQL,
though.)
> In my opinion, Date and Darwen and others have demonstrated serious
> problems with NULL, 3-VL, 4-VL and "n-VL as n approaches infinity" even
> when applied consistently. The inconsistent mess of SQL just makes a bad
> situation worse.
Agreed.
> I certainly commend Dr. Codd for attempting to tackle the problem, but I
> consider NULL the 'clay knapsack' of his work. Unlike the religious
> 'clay feet' from the Book of David, one can set NULL aside and the
> remainder of Codd's mathematics still stands on its own.
Agreed.
Marshall
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