Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
Date: Thu, 16 Jun 2005 18:27:01 GMT
>>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 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!
IMO that's not really a fair criticism. Whether you use the UR assumption or not, you always have to know what the columns of the relation(s) mean in order to understand what a query means. So in this case the querier can be assumed to know that the 'manager' column contains the manager of the department of the employee. If there is also another manager that is separately assigned to employees then the UR would probably contain a column 'dept-manager' and another column 'emp-manager' to distinguish the two concepts.
- Jan Hidders