Re: The Practical Benefits of the Relational Model

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