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From: L.Carl.Pedersen@dartmouth.edu (L. Carl Pedersen)
Newsgroups: comp.databases
Subject: Re: Relational Databases
Message-ID: <1991Jun7.205529.13222@dartvax.dartmouth.edu>
Date: 7 Jun 91 20:55:29 GMT
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In my opinion, if one is speaking carefully, there is no such thing as
a relational "database".  There are only relational database
*managers*.  In my opinion, a relational database manager (RDBMS) is a
program that implements something comparable in power to relational
algebra with persistent objects.  The objects represent relations, in
the mathematical sense,  and should be though of as such.  The
operations allow one to combine relations in various ways e.g., union,
subtraction, cartesian product, and to extract meaningful subsets of
those relations, or resultant relations, which are also relations.  An
important idea of these systems, often neglected, is that they operate
primarily on relations (aka tables, aggregates) rather than individual
rows (aka records).

I have heard people claim that relational systems are called same
because they allow one to relate data elements to each other.  That's
nonsense.  All database systems do that.  The term derives from the
mathematical concept of a relation, which is a set of tuples, the
elements of which are taken from several domain sets.  See Date's book
for a more rigorous definition.

All of the systems that I am aware of that I accept as "truly"
relational support something close to ANSII SQL - at least as a subset.
 This is an observation, not a definition of relational, but I think
it's safe to claim that if a product is truly relational, it should be
pretty easy to translate SQL into the language of that product (though
not necessarily the reverse).  

These remarks will probably draw a lot of flack.  So be it.