为什么我会得到重复的结果？ (DBpedia sparql查询)

Question

我以为我的查​​询很好，直到我看到结果有一些相同的事件出现了几次。谁能解释一下为什么？

查询：

PREFIX  dbo:  <http://dbpedia.org/ontology/>

SELECT ?event ?eventLabel

WHERE
  { ?event  a             dbo:MilitaryConflict ;
    dbo:date  ?date   .

FILTER ( ?date >= "1918-01-01"^^xsd:date && ?date <= "2018-12-31"^^xsd:date ).
FILTER(LANG(?eventLabel) = "nl").
OPTIONAL { ?event rdfs:label ?eventLabel.}}

结果：

http://dbpedia.org/sparql?default-graph-uri=http%3A%2F%2Fdbpedia.org&query=PREFIX++dbo%3A++%3Chttp%3A%2F%2Fdbpedia.org%2Fontology%2F%3E%0D%0A%0D%0A%23SELECT+++++%28+COUNT+%28DISTINCT+%3Fevent%29+AS+%3Fcnt+%29%0D%0ASELECT+%3Fevent+%3FeventLabel%0D%0A%0D%0AWHERE%0D%0A++%7B+%3Fevent++a+++++++++++++dbo%3AMilitaryConflict+%3B%0D%0A++++dbo%3Adate++%3Fdate+++.%0D%0A%0D%0A++++FILTER+%28+%3Fdate+%3E%3D+%221918-01-01%22%5E%5Exsd%3Adate+%26%26+%3Fdate+%3C%3D+%222018-12-31%22%5E%5Exsd%3Adate+%29.%0D%0A++++FILTER%28LANG%28%3FeventLabel%29+%3D+%22nl%22%29.%0D%0A+++OPTIONAL+%7B+%3Fevent+rdfs%3Alabel+%3FeventLabel.%7D%7D&format=text%2Fhtml&CXML_redir_for_subjs=121&CXML_redir_for_hrefs=&timeout=30000&debug=on&run=+Run+Query+

Answer 1

消除重复行--

SELECT ?event ?eventLabel

-- 应该做 --​​

SELECT DISTINCT ?event ?eventLabel