Re: Relation subset operators

From: <>
Date: Sun, 7 Jun 2009 10:57:10 -0700 (PDT)
Message-ID: <>

On Jun 7, 4:46 am, wrote:
> On 6 juin, 23:19, wrote:> On Jun 6, 11:43 am, wrote:
> Snipped
> > Now into the main topic -- set equality join.
> I am not sure that is actually the main topic.

You are right, I didn't read your message carefully. It was "subset" in the topic that confused me. None of your 5 questions require set join/relational division.

> In my perception, the
> specific problem which was adressed was the opportunity of using a new
> operator to simplify relation handling in the context of relational
> division. set equality is only but one aspect of operations that
> could potentially be expressed more effectively using such operator.

Well, all the questions decompose into the two parts: 1. Find a subset of sales by a certain criteria 2. Aggregate/group by it
Inlike step 2, Step 1 is very well understood from relational theory perspective. One can mix standard relational algebra operators, rearrange their order under certain rules, etc. As soon as you lump 1 and 2 together, the optimization becomes much less clear (unless it is just a shorthand/macro, which RDBMS engine expands into traditional notation).

> I am curious as to why you think that the idea of creating a new
> operator is *only* about *set containment*. As expressed the example
> provided (Question 1 to Question 8). The idea behind creating a new
> operator was actually that such operators allows a simpler but
> logically sound formulation of some operations that are tedious to
> express using traditional operators.

As I mentioned before there is no relational division anywhere in the examples 1-5. Yet you introduced the "/" symbol and call it relational division and later wrote an expression containing


This doesn't make any sense to me, how can you divide a relation CARSALE by an attribute salesman? Received on Sun Jun 07 2009 - 19:57:10 CEST

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