Aggregates and associated relational operators

Some time ago there has been already a small thread about broken symmetry among relational operators ("Naisetrac Product", which BTW should properly called "Tensor Product"). Recent discussion about "fundamental" aggregates satisfying identities like this:

sum(a union b) = sum(a) + sum(b)
max(a union b) = max(max(a),max(b))

prompts a related question:
Why union operator is so special that it has associated aggregation? Is there anything similar to aggregation that is missing? Received on Fri Sep 05 2003 - 17:49:37 CDT