- From: Steve Harris <steve.harris@garlik.com>
- Date: Wed, 9 Jun 2010 13:43:20 +0100
- To: Andy Seaborne <andy.seaborne@talis.com>
- Cc: Lee Feigenbaum <lee@thefigtrees.net>, Axel Polleres <axel.polleres@deri.org>, SPARQL Working Group <public-rdf-dawg@w3.org>
On 2010-06-09, at 10:23, Andy Seaborne wrote:
>
> On 09/06/2010 10:08 AM, Steve Harris wrote:
>>> which leads me to a fairly natural interpretation of
>>> >
>>> > SELECT ?s ?p
>>> > {
>>> > ?s ?p ?p
>>> > } GROUP BY ?s ?p
>>> >
>>> > as "null aggregation"
>> I don't understand the term "null aggregation".
>
> It is a term earlier in the thread to capture the idea that SELECT/GROUP with no aggregators mentioned fitted into the current framework with an implicit aggregator that did nothing.
This is captured in the current draft with:
"Definition: Group
Group evaluates a list of expressions against a solution sequence, producing a set of partial functions from keys to solution sequences.
The behaviour of Group is different when ExprList is empty.
Group((), Ω) = { 1 -> Ω }
Group(ExprList, Ω) = { ListEval(ExprList, μ) -> { μ' | μ' in Ω, ListEval(ExprList, μ) = ListEval(ExprList, μ') } | μ in Ω }"
http://www.w3.org/TR/2010/WD-sparql11-query-20100601/#aggregateAlgebra
"1" is just some arbitrary value that I plucked from thin air. I could have invented a new symbol, but that's arguably less clear. I guess the empty list would have been a good choice too.
- Steve
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Received on Wednesday, 9 June 2010 12:43:49 UTC