RE: The Third Manefesto

From: Lex de Haan <lex.de.haan_at_naturaljoin.nl>
Date: Wed, 20 Jul 2005 09:55:24 +0200
Message-Id: <20050720075522.9990F1D6EC0@turing.freelists.org>

Hi Henri,

I think the two issues are orthogonal. every relvar is based on a set of attributes, and every attribute is associated with a type. that is, a type of arbitrary complexity. so indeed, you can "hide" or encapsulate several employee properties into a single attribute if you like.

regardless how complicated you choose your relvar attributes, once that exercise is done you must assign (candidate) keys for that relvar. a candidate key is just a subset of the relvar heading (and the heading is the set of attributes) with the property that it uniquely identifies every tuple. well, a trivial key is the heading itself -- and if your relvar only has one attribute, that attribute *must* be the key ...

hope this helps,
kind regards,

Lex.

Steve Adams Seminar http://www.naturaljoin.nl/events/seminars.html

-----Original Message-----

From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Henry Poras
Sent: Wednesday, July 20, 2005 08:17
To: oracle-l_at_freelists.org
Subject: The Third Manefesto

I'm almost done reading Date's book. I have a bunch of questions and need to reread and think about a number of sections. There is one basic question, though, where I am convinced I must be missing something.

How do you apply Foreign Keys if relations are defined using relational types?

One example given by Date is logically representing employee data either in a relation variable (EMP) with attributes empno, ename, deptno, ... OR by defining a type emp(empno, ename, deptno, ...). This type is then the data type (domain) of an EMP relvar. The second relation is thus effectively a set of employee objects.

I see how joins can be done in the second case (use a built-in funtion THE_DEPTNO(emp) which will extract the deptno value from each tuple), but what about a foreign key on DEPTNO?

If I read this in the morning and the question is as unclear as I suppose it is, I'll expound on this.

Henry

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http://www.freelists.org/webpage/oracle-l Received on Wed Jul 20 2005 - 02:57:17 CDT