Re: object algebra

> 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.

> 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.

> Nor did it contain a discussion of operators
> which was what I thought you might think created nulls
> (it's OK if you don't think any of the operators
> create nulls - but it would help if you said so).

The operation (INSERT) was to be inferred from the analogy. A NULL occurs when reality can't provide a value required by a tuple. For example, a tuple might demand a person's name and eyeColor. But if the person has no eyes, we can either not create the tuple or create the tuple and incur a NULL for eyeColor. The NULL can subsequently be masked/patched with N/A, etc. Please see other postings for different views on this topic. Received on Fri Mar 05 2004 - 20:16:12 CET