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

"Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message news:MO9Rd.14260$fH2.1142837_at_phobos.telenet-ops.be...
> Mikito Harakiri wrote:
> >
> > So, what is the defining property of an aggregate?
>
> The defining property is that it is a function over bags. So if you want
> to base it on a binary operation this operation has to symmetric and
> associative.

Unless it's a text string aggregation (defined upon binary string concatenation operation). String concatenation is not symmetric, and string aggregation is defined over lists.

> > Next, is sum the only aggregate that can't be expressed by standard
means?
>
> Actually there is an infinite number of them, unless you want to limit
> the term artificially to those that are found in a certain query language.

Well, the argument that there are potentially infinite numbers of aggregate operators isn't really very convincing. When I open any math books, sigmas is the only aggregate I see. Others -- products and lower/upper bounds -- are rarities.

> > If it is, then don't you think that justification for an aggregation
syntax
> > is too thin?
>
> If the users can understand and use it easily and it can be implemented
> efficiently then that is all the justification that is needed.
> Mathematical elegance is only circumstantial evidence here.

Citing David Cressey: "Can you trust relational language specification to the users?" Received on Fri Feb 18 2005 - 01:12:27 CET