On Tue, Jul 30, 2013 at 12:58 PM, Joshua TAYLOR <[email protected]> wrote:
> This query
>
> prefix : <http://example.org/>
> select ?x ?y where {
> values (?x ?y) {
> (1 2)
> (1 UNDEF )
> }
> }
>
> produces
>
> ---------
> | x | y |
> =========
> | 1 | |
> | 1 | 2 |
> ---------
>
> because in the ordering, the UNDEF ?y comes before the 2 ?y. As such,
> I'd expect that if I were to group by ?x and have the aggregate set
> for ?y = { UNDEF, 2 } and select max(?y) that I should get 2.
> However, this query
>
> order by ?x ?y
> prefix : <http://example.org/>
> select ?x (max(?y) as ?maxY) where {
> values (?x ?y) {
> (1 2)
> (1 UNDEF)
> }
> }
> group by ?x
> order by ?x
>
> which does just that, produces:
>
> ------------
> | x | maxY |
> ============
> | 1 | |
> ------------
>
> Am I misunderstanding how max [1] works? The spec says that max makes
> use of the ORDER BY ordering, so I'm surprised by these results. Have
> I missed something?
>
> Thanks,
> //JT
>
> [1] http://www.w3.org/TR/sparql11-query/#defn_aggMax
And here's minimal working code, in case you want to reproduce this.
Sorry, I meant to include this in the first email:
import com.hp.hpl.jena.query.QueryExecutionFactory;
import com.hp.hpl.jena.query.ResultSetFormatter;
import com.hp.hpl.jena.rdf.model.Model;
import com.hp.hpl.jena.rdf.model.ModelFactory;
public class GroupAndIf {
final static String orderQuery = ""+
"prefix : <http://example.org/> \n" +
"select ?x ?y where {\n" +
" values (?x ?y) {\n" +
" (1 2)\n" +
" (1 UNDEF )\n" +
" }\n" +
"}\n" +
"order by ?x ?y" +
"";
final static String groupQuery = "" +
"prefix : <http://example.org/> \n" +
"select ?x (max(?y) as ?maxY) where {\n" +
" values (?x ?y) {\n" +
" (1 2)\n" +
" (1 UNDEF)\n" +
" }\n" +
"}\n" +
"group by ?x\n" +
"order by ?x\n" +
"";
public static void main(String[] args) {
final Model model = ModelFactory.createDefaultModel();
System.out.println( orderQuery );
System.out.println( groupQuery );
// Note that in the output, ordered by ?x ?y, the UNDEF < 2
ResultSetFormatter.out( QueryExecutionFactory.create(
orderQuery,
model ).execSelect() );
// Yet when we group by ?x and ask for (max(?y) as ?maxY), we
get an
undefined ?maxY.
ResultSetFormatter.out( QueryExecutionFactory.create(
groupQuery,
model ).execSelect() );
}
}
--
Joshua Taylor, http://www.cs.rpi.edu/~tayloj/
