On 2011-04-22, at 06:18, Jeen Broekstra wrote:
>
> Hi all, (Cc Ruslan),
>
> Ruslan and I are currently working on conformance testing for Sesame's
> implementation of SPARQL 1.1 query, and there is a case where I am not 100%
> sure what the expected behavior is. This case involves a query that uses a
> GROUP_CONCAT aggregate, a grouping, and an order by solution modifier. Before
> I start pestering the SPARQL working group I'd like to hear some other
> developers' thoughts.
>
> All SPARQL aggregate operators are defined to work on multisets. This means
> that by default, the input of an aggregate does not have any prescribed
> order. For most aggregates this is irrelevant anyway, but for GROUP_CONCAT it
> does make a difference. Consider the following example data:
>
> :org1 :affiliates :p1, :p2 .
> :org2 :affiliates :p3, :p4 .
>
> :p1 :name "John" .
> :p2 :name "Paul" .
> :p3 :name "Ringo" .
> :p4 :name "George" .
>
> I want to produce a query that gives me concatenated names per organisation.
> Each concatenated string should have the names in alphabetical order.
>
> My initial thought was that this query would do the trick:
>
> SELECT ?org (GROUP_CONCAT(?name) as ?names)
> WHERE {?org :affiliates ?p. ?p :name ?name }
> GROUP BY ?org
> ORDER BY ASC(str(?name))
>
> Expected result:
>
> ?org ?names
> --------------
> :org1 "John Paul"
> :org2 "George Ringo"
>
> However, looking at the SPARQL 1.1 query spec, I think this is not a
> guaranteed result: as far as I can tell the solution modifier ORDER BY should
> be applied to the solution sequence _after_ grouping and aggregation, so it
> can not influence the order of the input for the GROUP_CONCAT. This would
> mean that for the above query, the result could equally well be:
That's correct.
> ?org ?names
> --------------
> :org1 "Paul John"
> :org2 "George Ringo"
>
> or indeed any other permutation of name concatenations.
>
> I have thought about using some subquery to solve the problem, but since
> SPARQL defines the input of an aggregate operator explicitly as a _set_, I am
> not even sure that would work: as far as I can tell a SPARQL engine has no
> obligation to preserve input order when evaluating aggregate operators.
>
> Two questions:
>
> 1. is the above correct?
> 2. is there any other way in SPARQL 1.1 to enforce ordering on a GROUP_CONCAT?
>
> In relation to question 2, I note that in MySQL, the standard SQL
> group_concat operator (on which, I assume, the SPARQL operator has been
> based) has been extended to include an ordering clause as an argument to the
> group_concat function itself. See
> http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_group-concat.
Yup, that's where the inspiration came from.
- Steve
