Re: What is Aggregation? Re: grouping in tuple relational calculus

Mikito Harakiri wrote:
> So, what is the defining property of an aggregate? Is it
>
> 2. Incremental cumulative of some associative binary operation:
>
> ((3 + 2) + 5) + ...
>
> (Ignoring the obvious exceptions like avg since they are redundant).

It would have to be commutative as well. Unless you want your aggregate to require a sort order I suppose. In which case you could drop associativity too!

> Next, is sum the only aggregate that can't be expressed by standard means?
> If it is, then don't you think that justification for an aggregation syntax
> is too thin?

what about a domain of a collection of "sets", with binary operations "union" and "intersection" that could be generalised to n-ary operations?

For example consider the domain consisting of the values:

({1,2,3,4}, {1,2,3}, {1,2})

I don't think the binary operation has to have an identity element or inverses does it? It just needs to be associative, commutative, and closed for the general case.

Paul. Received on Fri Feb 18 2005 - 00:49:14 CET