From: Jim Clifford <[email-masked]>
Date: Mon, 24 May 93 14:46:37 EDT

This is in reply to the latest message from Angelo, Elisa and Barbara (AEB).

> It was said that our two employees are identified by ED and DI,
> respectively. Are those a sort of surrogates or not?

As I undersatnd from the Benchmark document and, more especially, from
the discussions that took place via e-mail on the issue of the
"instance," the terms ED and DI are meant to indicate the actual
people in the real world. Given that they are not (aand can not) be
physically present --- in which case we would point to them --- we
have no other mecahnism to communicate with one another about them
than to use some "words' in our un-formalized language (Englsih) when
we communciate with each other.

The database schema has been fixed for some time now. There is no
mention in it about any such things as "surrogates". The Emp ralation
therefore has all and only those attributes indicated, namely : Name,
Salary, Dept, Gender, and D-birth.

The remainder of the points in the e-mail from AEB seem to me to
presuppose a particular representation for the instance.

> For instance, in the given examples, employee names are allowed to
> change over time and it is assumed that the system is responsible to
> mantain the links between the tuples referring to the same real-world
> entity. However there is no mean to distinguish in the query result
> between a person who has changed name from two different persons. If
> we consider the query Q~2.1.1 ``{\em Find the names of employees that
> have been in a department named Toy for a shorter period than has DI}\
> ", how can we establish that Ed and Edward are the same person, rather
> than being two different persons?

Depending upon how one imagines the instance to be represented
(grouped -vs- ungrouped, in particular), the
issue raised here of how the model
"can establish that Ed and Edward are the same person, rather
 than being two different persons" may or may not be a problem.

Comments are welcome.

--Jim Clifford--

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Jim Clifford                                     [email-masked]
Associate Professor                              TEL: (212) 998-0803
Department of Information Systems                FAX: (212) 995-4228
Leonard N. Stern School of Business
New York University
Management Education Center
44 West 4th Street, Suite 9-74
New York, NY 10012-1126
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