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

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.

> 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.

> 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.

• Jan Hidders