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Home -> Community -> Usenet -> comp.databases.theory -> Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

From: Jon Heggland <heggland_at_idi.ntnu.no>
Date: Thu, 16 Jun 2005 15:12:13 +0200
Message-ID: <MPG.1d1b991d12c173e5989699@news.ntnu.no>


In article <42b16163$1_at_news.fhg.de>, savinov_at_host.com says...
> Jan Hidders schrieb:
> > For a very short explanation and a link to Jeffrey Ullman's sheets:
> >
> > http://app.deklarit.com/kb/article.aspx?id=10038&cNode=8J8X2Y
>
> I find the problems raised in UR model very similar to those motivating
> COM. However, the provided solution is hardly acceptable (it is
> typcially relational). In COM we have advantages of UR model but the
> solution is based on other principles.
>
> In particular, the query
>
> select office_id
> from Offices
> where Managers.name = 'sally'

I looked at Ullman's slides and found a strange thing. I quote:

Suppose we have relations ED, EO, EP, and DM, connecting employees to departments, phones, and offices, respectively, and departments to managers. [...]
Consider a query "find the offices of employees managed by Sally."

(End quote)

Note that the relations do not say that *employees* have managers, just that *departments* do. Is it the case that an employee in a given department is always managed by the manager of that department? It is possible, even probable, but the database (as presented) does not say!

Thus, the query is really underspecified (or invalid, even), and the presented solution is based on an assumption that may be wrong. Just an observation, but I think it is worth noting.

-- 
Jon
Received on Thu Jun 16 2005 - 08:12:13 CDT

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