From: Lee Feigenbaum <lee@thefigtrees.net>

Date: Sat, 25 Jun 2011 11:18:39 -0400

Message-ID: <4E05FC4F.6040407@thefigtrees.net>

To: SPARQL Working Group <public-rdf-dawg@w3.org>

Date: Sat, 25 Jun 2011 11:18:39 -0400

Message-ID: <4E05FC4F.6040407@thefigtrees.net>

To: SPARQL Working Group <public-rdf-dawg@w3.org>

On the surface, Jeen's reasoning makes sense to me. Steve, did we/you consider defining SUM instead of "+" instead of in terms of op:numeric-add? Lee -------- Original Message -------- Subject: SUM aggregate operator and non-numeric literals Resent-Date: Thu, 23 Jun 2011 01:05:51 +0000 Resent-From: public-rdf-dawg-comments@w3.org Date: Thu, 23 Jun 2011 13:05:10 +1200 From: Jeen Broekstra <jeen.broekstra@gmail.com> To: public-rdf-dawg-comments@w3.org Hi DAWG, The current definition of SUM (section 18.4) is as follows : ==begin quote== Definition: Sum numeric Sum(multiset M) The Sum set function is used by the SUM aggregate in the syntax. Sum(M) = Sum(ToList(Flatten(M))). Sum(S) = op:numeric-add(S1, Sum(S2..n)) when card[S] > 1 Sum(S) = op:numeric-add(S1, 0) when card[S] = 1 Sum(S) = 0 when card[S] = 0 In this way, Sum({1, 2, 3}) = op:numeric-add(1, op:numeric-add(2, op:numeric-add(3, 0))). ==end quote== Given that the definition of SUM is directly in terms of the op:numeric-add XPath function, it follows that it can only be applied on numeric literals. Therefore, any SUM that aggregates over a set of values that contains a non-numeric type will result in a type error. Not even an extension of the SPARQL operator table in section 17.3 will help, as SUM is not defined in terms of those operators. In other words, if we have the following data: :a rdf:value "1" . :a rdf:value "2"^^xsd:integer . :b rdf:value "3"^^xsd:integer . And the following query: SELECT (SUM(?val) as ?value) WHERE { ?a rdf:value ?val . } GROUP BY ?a The result will be always a type error. I would argue that having the same extensibility mechanisms available for SUM as we have for, for example, the + operator would be preferable. That way, implementations wanting to offer a more forgiving version of the SUM operator (one which silently ignores the non-numerics, for example), could do so while staying spec-compliant. Regards, JeenReceived on Saturday, 25 June 2011 15:19:08 UTC

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