Re: object algebra
Date: Sun, 07 Mar 2004 05:14:57 GMT
Message-ID: <ldy2c.22867$yZ1.3107_at_newsread2.news.pas.earthlink.net>
Neo wrote:
>Jonathan Leffler wrote:
>>Please can you explain the definition of a relation which you think
>>means that the RDM must create nulls? And can you give a reference to
>>where the version of the RDM you are using or assuming is defined? It
>>doesn't seem to be the same as the version(s) I'm used to.
>
> I am using the definition which matches C. J. Date's definition in his
> book "An Introduction to Database Systems", 6th Ed, ISBN
> 0-201-54329-X, in Chapter 4, section 3, under the heading of Relation.
> A relation consist of two parts, a heading and a body. The heading
> consists of a fixed set of attributes. The body consists of a set of
> tuples. Each tuple consists a set of values corresponding to the
> heading.
And where in that chapter are nulls described? I dug out my copy of the book (it was in peaceful retirement since I have the 7th and 8th editions for current references), and an admittedly quick scan did not show me even a footnote mentioning nulls, nor any mention of missing information - which is where nulls appear. Indeed, I would not expect to find any discussion of nulls in that section of the book until chapter 8, which is on SQL.
>> Your previous answer contained no definition of a relation which >> explains why nulls are an integral part of the RDM.
> The defintion of a relation and how NULLs are incurred was to be
> inferred from the analogy about Rover callecting data on Mars.
A rather less than compelling example, IMO - and the main inference I drew was that you are not very good at explaining what you're talking about.
> The operation (INSERT) was to be inferred from the analogy. A NULL
> occurs when reality can't provide a value required by a tuple.
This is not compelling. If you can't provide a value for a column, then you can't insert the row - in a relational DBMS (as distinct from, or opposed to, an SQL DBMS).
> Please see other postings for different
> views on this topic.
No, I'm about to put this thread on my 'ignored' list.
-- Jonathan Leffler #include <disclaimer.h> Email: jleffler_at_earthlink.net, jleffler_at_us.ibm.com Guardian of DBD::Informix v2003.04 -- http://dbi.perl.org/Received on Sun Mar 07 2004 - 06:14:57 CET