Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
Date: Thu, 16 Jun 2005 15:47:09 +0200
Jon Heggland schrieb:
> In article <42b16163$1_at_news.fhg.de>, firstname.lastname@example.org 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.
This problem is obvious for me. In this case we have a kind of zigzag query (we move up and down in the hierarchy several times). Normal inference is where we propagate constraints down (to the universal relation) and then aggregate it up to the target.
There is also another problem. Sometimes there are alternative (not
unique) paths along which we can propagate constraints. In this case the
database also cannot help but it is not its fault. The model simply does
not define what path to choose (path is used in terms of COM).
In UR it is a serious drawback bcause it is based on relational model
(its spirit and fundamental assumptions), it is actually a complement to
the RM. We need to look at the data differently in order to solve the
In UR it is a serious drawback bcause it is based on relational model (its spirit and fundamental assumptions), it is actually a complement to the RM. We need to look at the data differently in order to solve the problem.
-- alex http://conceptoriented.comReceived on Thu Jun 16 2005 - 15:47:09 CEST