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

"Paul" <paul_at_test.com> wrote in message news:42152d7a$0$35759$ed2619ec_at_ptn-nntp-reader02.plus.net...
> 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!

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.)

> 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.

Neutral element is required. Otherwise what is your answer on an empty set/bag/list? Received on Fri Feb 18 2005 - 01:30:56 CET