Negation allows to refer to conditions where an answer exists or does not exists
.
For instance making a small select statement on a database with three individuals in it for which not all the name is given but the type Person is defined.
If one wants to select all person from a database where one specific attribute is not available then one can do this with the filter expression NOT EXISTS
, where one can say FILTER NOT EXIST the name, or a Triple that has a foaf name in it. With such a simple filter NOT EXISTS
one can filter out results and therefore give specific things that should not be provides in the results.
Data:
@prefix : <http://example/>] .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>]
@prefix foaf: <http://xmlns.com/foaf/0.1/>] .
:hevok rdf:type foaf:Person .
:hevok foaf:name "Hevok" .
:eva rdf:type foaf:Person .
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-nx#>]
PREFIX foaf: <http://xmlns/foaf/0.1/>]
SELECT ?person
WHERE
{
?person rdf:type foaf:Person .
FILTER NOT EXISTS { ?person foaf:name ?name }
}
Result
?person
eva
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