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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: Fri, 17 Jun 2005 12:26:13 +0200
Message-ID: <MPG.1d1cc3b086296db898969d@news.ntnu.no>


In article <42b28fe7$1_at_news.fhg.de>, savinov_at_host.com says...
> Let's take three tables E, D, and M and two subtables ED and DM. If we
> impose constraints on M then we are not able to infer anything on E. The
> reason is formally clear: ED and DM are independent (orthogonal) because
> they do not have a common subtable (a formal criterion). This means that
> the canonical semantics (or primitive semantics) is represented as a sum
> of two tables ED and DM without intersection like this one:
>
> emp1, dept1, null, null
> emp2, dept2, null, null
> null, null, dept1, mng1
> null, null, dept2, mng2

Is this better than the corresponding notion in the RM, namely the join (logical and) of ED and DM?

emp1, dept1, mng1
emp2, dept2, mng2

By the way, what kind of construct is your 4x4 table in your terminology?

> It consists of two isolated sections so it is not a fault of the
> database that it cannot infer anything about employees given information
> about managers. We need somehow connect two department columns in the
> middle. In the RM it is not a problem because everything is done
> manually without any rules,

No, no, no. In the RM, this is done formally, with very definite rules indeed. ED and DM are not independent---at least not in the "unrelated" sense---they both involve departments. The database knows this (if it is properly designed), and it can suggest it to the user, if necessary.

-- 
Jon
Received on Fri Jun 17 2005 - 05:26:13 CDT

Original text of this message

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