如何在SPARQL中执行COUNT

Question

鉴于这个非常简单的模型:

@prefix :        <http://example.org/tags#> .
@prefix owl:     <http://www.w3.org/2002/07/owl#> .
@prefix rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs:    <http://www.w3.org/2000/01/rdf-schema#> .
@prefix xsd:     <http://www.w3.org/2001/XMLSchema#> .

:tag  rdf:type rdf:Property .

:item1
      rdf:type owl:Thing ;
      :tag    "a"^^xsd:string .

:item2
      rdf:type owl:Thing ;
      :tag    "a"^^xsd:string , "b"^^xsd:string .

:item3
      rdf:type owl:Thing ;
      :tag    "a"^^xsd:string , "b"^^xsd:string , "c"^^xsd:string .

我想获得每个项目的列表和标签数量:

item  tagCount
===== ========
item1 1
item2 2
item3 3

这是我的查询:

SELECT ?item (count(?tag) as ?tagcount)
WHERE {
     ?item :tag ?tag
}

然而它正在回归:

item  tagCount
===== ========
      6

从我所读到的,这应该工作.我正在使用Jena 2.6.4

Answer 1

我没试过这个,但尝试添加GROUP BY ?item到查询的末尾.我认为没有GROUP BY它只计算总行数.