DISTINCT changes order of results

[RicardoUsbeck]

Hi,

for the query below, the DISTINCT operator (try to leave that out) changes the result set, although the answer semantic should be the same:

PREFIX dbo: <http://dbpedia.org/ontology/>
PREFIX res: <http://dbpedia.org/resource/>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
SELECT DISTINCT ?uri
WHERE {
        ?x rdf:type dbo:Album .
        ?x dbo:artist res:Elvis_Presley .
        ?x dbo:releaseDate ?y .
        ?x dbo:recordLabel ?uri .
}
ORDER BY ASC(?y) 
LIMIT 1

Obviously the DISTINCT operator is applied before the LIMIT operator but it changes the order. I am not sure whether this is a bug or a feature.

[kidehen]

On 10/2/17 2:32 AM, Ricardo Usbeck wrote:

Hi,

for the query below, the DISTINCT operator (try to leave that out) changes the result set, although the answer semantic should be the same:

|PREFIX dbo: http://dbpedia.org/ontology/ PREFIX res: http://dbpedia.org/resource/ PREFIX rdf: http://www.w3.org/1999/02/22-rdf-syntax-ns# SELECT DISTINCT ?uri WHERE { ?x rdf:type dbo:Album . ?x dbo:artist res:Elvis_Presley . ?x dbo:releaseDate ?y . ?x dbo:recordLabel ?uri . } ORDER BY ASC(?y) LIMIT 1 |

Obviously the DISTINCT operator is applied before the LIMIT operator but it changes the order. I am not sure whether this is a bug or a feature.

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Virtuoso is a Quad Store so please repeat using:

|PREFIX dbo: http://dbpedia.org/ontology/ PREFIX res: http://dbpedia.org/resource/ PREFIX rdf: http://www.w3.org/1999/02/22-rdf-syntax-ns# SELECT DISTINCT ?uri ?g WHERE { GRAPH ?g { ?x rdf:type dbo:Album . ?x dbo:artist res:Elvis_Presley . ?x dbo:releaseDate ?y . ?x dbo:recordLabel ?uri . } } ORDER BY ASC(?y) LIMIT 1|

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[RicardoUsbeck]

Still, if I use this query with DISTINCT the result is http://dbpedia.org/resource/Pickwick_Records If I use it without DISTINCT the result is http://dbpedia.org/resource/RCA_Victor

Where RCA Victor is semantically correct

[JervenBolleman]

PREFIX dbo: <http://dbpedia.org/ontology/>
PREFIX res: <http://dbpedia.org/resource/>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
SELECT DISTINCT ?uri
WHERE {
        ?x rdf:type dbo:Album .
        ?x dbo:artist res:Elvis_Presley .
        ?x dbo:releaseDate ?y .
        ?x dbo:recordLabel ?uri .
}
GROUP BY ?y ORDER BY ASC(?y) 
LIMIT 1

Will give you the results you expect. It does seem that in this case the query engine does not preserve the fact that it must first order, then project variables and only then remove duplicates as per standard.