Re: Date's First Great Blunder
Date: 21 Apr 2004 14:37:23 -0700
Message-ID: <4b45d3ad.0404211337.7d87cd7f_at_posting.google.com>
> > If NULLs are not necessary and increase complexity too much, why doesn't
> > Date drop the chapter on NULL/Missing Information?
>
> From which book?
An esoteric series of books titled "An Introduction to Database Systems". It is Chapter 20 in his 6th Edition.
> Anyway he has to deal that NULLs exist on SQL.
Even if one builds a RDM db without an SQL layer, the db will still generate NULLs in certain circumstances. A tuple's attributes demands values specified by the relation's header. But reality says I don't care what your relation header demands, I can't give you that value. Reality can say this for almost any attribute of a tuple. In such a case, RDM can 1) refuse to add the tuple 2) add the tuple but create a NULL for the "missing info" 3) change the schema and seperate the column to a new table to avoid the NULL (which can become impractical in many cases).
> Furthermore, NULLs do not equal missing information.
True, NULL is what RDM creates when reality can't/won't provide a value demanded a tuple's attribute. (Reality generally wins this argument).
> He does deal with missing information quite nicely, with the proposal to
> include special values into each domain needing them.
> Looks like you never understood the special values proposal.
> > Per Date, Intro to Db Sys, "3VL suffers from the very serious
> > ('showstopper') problem that it does not match reality - that is, results
> > that are correct according to 3VL are sometimes incorrect in the real
> > world. NULLs and 3VL undermine the entire foundation of the relational
> > model."
>
> do think he's mixing things here.
Date is correct on this topic. Received on Wed Apr 21 2004 - 23:37:23 CEST