Re: The Practical Benefits of the Relational Model
From: Jan.Hidders <hidders_at_hcoss.uia.ac.be>
Date: 7 Oct 2002 17:22:08 +0200
Message-ID: <3da1a6a0$1_at_news.uia.ac.be>
Date: 7 Oct 2002 17:22:08 +0200
Message-ID: <3da1a6a0$1_at_news.uia.ac.be>
In article <bdf69bdf.0210050540.6dd309d7_at_posting.google.com>,
Mikito Harakiri <mikharakiri_at_yahoo.com> wrote:
>> Therefore, the pairs of dual relational operations are:
>
>> 1. Intersection is dual to Cartesian Product.
>> 2. Union is dual to Naisetrac Product.
>> 3. Selection is dual to Projection.
>
>Correction:
>
>Intersection = Cartesian Product * Selection
Er, I would say you also need a projection at the end. If columns are labeled you simulate the intersection with only a join.
>Union = Naisetrac Product * Projection
So that wouldn't be really symmetric then, would it?
- Jan Hidders