我想运行以下简单的测试查询：
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX vcard: <http://www.w3.org/2001/vcard-rdf/3.0#>
SELECT ?givenName ?name_count ?temp
WHERE
{ BIND(if(( ?name_count = 2 ), "just two", "definitely not 2") AS ?temp)
{ SELECT DISTINCT ?givenName (COUNT(?givenName) AS ?name_count)
WHERE
{ ?y vcard:Family ?givenName }
GROUP BY ?givenName
}
}
我要查询的图是来自教程https://jena.apache.org/tutorials/sparql_data.html的图：
@prefix vCard: <http://www.w3.org/2001/vcard-rdf/3.0#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
<http://somewhere/MattJones/> vCard:FN "Matt Jones" .
<http://somewhere/MattJones/> vCard:N _:b0 .
_:b0 vCard:Family "Jones" .
_:b0 vCard:Given "Matthew" .
<http://somewhere/RebeccaSmith/> vCard:FN "Becky Smith" .
<http://somewhere/RebeccaSmith/> vCard:N _:b1 .
_:b1 vCard:Family "Smith" .
_:b1 vCard:Given "Rebecca" .
<http://somewhere/JohnSmith/> vCard:FN "John Smith" .
<http://somewhere/JohnSmith/> vCard:N _:b2 .
_:b2 vCard:Family "Smith" .
_:b2 vCard:Given "John" .
<http://somewhere/SarahJones/> vCard:FN "Sarah Jones" .
<http://somewhere/SarahJones/> vCard:N _:b3 .
_:b3 vCard:Family "Jones" .
_:b3 vCard:Given "Sarah" .
现在的问题是用耶拿运行它：
Query query = QueryFactory.create(theAboveQueryAsString);
QueryExecution qexec = QueryExecutionFactory.create(query, theAboveGraphmodel);
ResultSet execSel = qexec.execSelect();
ResultSetRewindable results = ResultSetFactory.copyResults(execSel);;
ResultSetFormatter.out(System.out, results, query);
在控制台中发出以下结果：
----------------------------------
| givenName | name_count | temp |
==================================
| "Smith" | 2 | |
| "Jones" | 2 | |
----------------------------------
将温度值设为null。
另一方面，在Ontotext GraphDb环境中的相同图形上运行相同的查询，我得到正确的结果（保存为CSV）：
givenName | name_count | temp
------------------------------------
Jones | 2 | just two
Smith | 2 | just two
ARQ引擎中可能有错误还是我遗漏了一些东西？
提前致谢。
我正在使用jena-arq 3.12.0
Java（TM）SE运行时环境（内部版本1.8.0_181-b13）
Eclipse版本版本：2019-06（4.12.0）
最佳答案
BIND和子选择之间存在联接。在完成连接之前计算连接步骤的参数。因此，对BIND进行评估，对子选择进行单独评估，并将结果合并。未在BIND分配中设置？name_count。如果您在子选择之后将其移动，它将应用于子选择的结果。
BIND将绑定添加到模式结果之前。
(base <http://example/base/>
(prefix ((rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>)
(vcard: <http://www.w3.org/2001/vcard-rdf/3.0#>))
(project (?givenName ?name_count ?temp)
(join
(extend ((?temp (if (= ?name_count 2) "just two" "definitely not 2")))
(table unit))
(distinct
(project (?givenName ?name_count)
(extend ((?name_count ?.0))
(group (?givenName) ((?.0 (count ?givenName)))
(bgp (triple ?y vcard:Family ?givenName))))))))))
在这里，
(extend...)是(join...)的两个参数之一。 (table unit)是BIND之前的“无”。如果放在后面，则代数为：
(base <http://example/base/>
(prefix ((rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>)
(vcard: <http://www.w3.org/2001/vcard-rdf/3.0#>))
(project (?givenName ?name_count ?temp)
(extend ((?temp (if (= ?name_count 2) "just two" "definitely not 2")))
(distinct
(project (?givenName ?name_count)
(extend ((?name_count ?.0))
(group (?givenName) ((?.0 (count ?givenName)))
(bgp (triple ?y vcard:Family ?givenName))))))))))
并且
extend（来自BIND语法）正在处理子查询的(distinct ...。