SPARQL different operator - sparql

Question

SPARQL Query
I have some SPARQL query shown below:
SELECT DISTINCT ?name1
WHERE {
GRAPH <blabla>
{
?k swrc:author ?x .
?x foaf:name ?name1 .
} .
GRAPH <blabla2>
{
?l swrc:author ?y .
?y foaf:name ?name2 .
} .
FILTER(?x != ?y) .
}
I want to get the names that exist only in the first graph blabla.
Problem
Counter intuitively I get some names that actually belong to the intersection. This happens because b (of set A) = b (of set B)?
Question
What are exactly the semantics of != ? How can I surpass this problem?

Answer 1

The semantics of != are exactly that its left argument is not equal to its right argument. But a FILTER is evaluated for every possible combination of values - so the query as you formulated it will return all name-values of ?x for which some value of ?y is not equal to it.
If you want to get back only name-values of ?x for which all values of ?y are not equal to it, you should be using a NOT EXISTS clause:
SELECT DISTINCT ?name1
WHERE {
GRAPH <blabla>
{
?k swrc:author ?x.
?x foaf:name ?name1.
}
FILTER NOT EXISTS {
GRAPH <blabla2>
{
?l swrc:author ?x.
}
}
}
Note that using this approach you can actually get rid of variable ?y altogether: you change the condition to just check that author ?x as found in the first graph does not also occur in the second graph.