I have seen reference (here and elsewhere) to the "common knowledge"
that the relational algebra is equivalent in power to the first
order logic.
I would like to understand this proof, but I can't find any

As a way to describe a problem domain with end users, why hasn't ORM
taken off?  From what I have read of it, it seems that it would be the
place to start, then tweak on ER layer...

I recently started trying to understand the relational model.  I took a
DBMS class (undergrad) about five years ago...more recently I have been
studying C. J. Date's 8th edition of 'Database Systems.'
I would like to look at what recent work has been done in either

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Glossary 0.0.5:     "We have many future joys."
july 10, 2005           The mythical man-month - Fred Brooks
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<quote http://www.inconcept.com/jcm/may2005/David.html>
This results in XML query languages having to be procedurally supplied
with hierarchical semantic logic causing significant difficulty in
specifying hierarchical queries that operate across multiple legs of

I know a number of database modelers try to avoid NULLs in their models
because of their "unpredictability" and I believe there is merit in
this approach. Is the ideal way to do this to move the optional
attribute into it's own table? For example:

I'm seeking some help on a rather basic and trivial question.
Is there a case where a relation's extension proves to be symmetric and
transitive, but the relation is not an equivalence relation?  In other
words, can a relation be symmetric and transitive, but not be

When creating sub-entities should you put a column in the root table
saying what kind of sub-entity it is, or is the presence of a row in
the more specific table? For example:
CREATE TABLE dbo.Clients

In chapter 4 or The Third Manifesto, D&D define a "new relational
algebra."
This algebra includes two operations named "<AND>" and "<OR>". (They
use some weird triangle characters which I'm approximating with <>.)

Could some post a RM script equivalent to the following xrdb script?
// Create type person and make it an item of main directory.
// Note: "inst" is short for instance.
(CREATE type inst *person  & dir item it)