Re: Lucid statement of the MV vs RM position?

Bob Badour wrote:
> Jon Heggland wrote:

>> That said, I guess GROUP could also be defined as a SUMMARIZE with an
>> aggregate operator (or "summary"; TTM distinguishes between them) that
>> computes the RVA value based on a set of attributes. That aggop/summary
>> would be a bit non-standard, though, since it would need take a variable
>> number of arguments, not just one. That still wouldn't make GROUP an
>> aggregate operator, though.

Maybe I was unclear. TTM has such an aggregate operator; it is called UNION. You could define GROUP as SUMMARIZE ... ADD (UNION(...) AS RVA)---provided you have this mechanism for "converting" a set of attributes to a singleton relation---but GROUP is still not an aggregate operator. UNION is. In other words, UNION is to (eh...) UNION as SUM is to +.

> I don't see this as any different than performing a type conversion
> before performing the addition for SUM.

I think we are just quibbling over Tutorial D syntax, or over which operators should be defined in terms of which others. R GROUP ({X} AS Y) is a valid expression; R SUM (X AS Y) is not.
SUMMARIZE R BY { ALL BUT X } ADD (SUM(X) AS Y) is a valid expression; SUMMARIZE R BY { ALL BUT X } ADD (GROUP(X) AS Y) is not.

> Aggregate operations are defined using an identity element and a
> repeated operation or as an expression on other aggregate operations.

Thanks! Source?

-- 
Jon