# Re: Domains as relations

Date: 10 Oct 2002 14:58:18 +0200

Message-ID: <3da5796a$1_at_news.uia.ac.be>

In article <51d64140.0210100319.179421f8_at_posting.google.com>,
Paul <pbrazier_at_cosmos-uk.co.uk> wrote:

*>
*

>I've been thinking about expressing aggregates in purely relational

*>language and I'm not sure if it is possible.
*

Excellent question. In general you cannot. It is for instance well known that just adding counting (the simplest of aggregation in some sense, and something you can simulate if you have SUM) already adds expressive power to the relational algebra. For example, if you have counting you can ask the query "are there an even number of tuples in this relation", which is not possible in just the relational algebra.

>[...] But I think this may be a problem just with the query language rather

*>than the underlying relational theory.
*

Exactly. The relational algebra should be more regarded as a lowerbound than as an upperbound.

- Jan Hidders