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

"Paul" <paul_at_test.com> wrote in message news:4215b218$0$68538$ed2619ec_at_ptn-nntp-reader01.plus.net...
> Mikito Harakiri wrote:
> > Can you give an example of an aggregate that violates accociativity?
(For
> > commutativity it's simple: just aggragate elements into a list, or
> > concatenate a string.)
>
> How about the function f(a,b) -> a*b + 1 ?
>
> It's a commutative but not associative binary operator on some numerical
> domain (e.g. integers). You could aggregate this in the usual way but
> the aggregate would depend on the order that you did it.
>
> Maybe it's not one you would use in practice but I'm sure there must be
> examples that are more realistic - all you need is a non-associative
> binary operator - maybe some matrix multiplication or something?

Matrix multiplication is associative.

If you drop associativity, then the underlying relation structure is no longer a set/list/bag. It seems to be a binary tree. Received on Fri Feb 18 2005 - 18:52:27 CET