Title: ARQ - Negation

Standard SPARQL can perform negation using the idiom of
`OPTIONAL`/`!BOUND`. It is inconvenient and can be hard to use as
complexity increases. ARQ supports additional operators for
negation.

This is syntactic extension and is available if the query is parsed
with language `Syntax.syntaxARQ`.

    # Names of people who have not stated that they know anyone
    PREFIX foaf: <http://xmlns.com/foaf/0.1/>
    SELECT ?name
    WHERE { ?x foaf:givenName  ?name .
        OPTIONAL { ?x foaf:knows ?who } .
        FILTER (!BOUND(?who))
    }

In ARQ, there is the `NOT EXISTS` operator (`UNSAID` is an alias
for `NOT EXISTS`) which acts at the point in the query where it is
written. It does not bind any variables but variables aready boudn
in the query will have their bound value.

    # Names of people who have not stated that they know anyone
    PREFIX foaf: <http://xmlns.com/foaf/0.1/>
    SELECT ?name
    WHERE { ?x foaf:givenName ?name .
        NOT EXISTS { ?x foaf:knows ?who }
          }

There is also an `EXISTS` operator.

    # Names of people where it is stated that they know at least one other person.
    PREFIX foaf: <http://xmlns.com/foaf/0.1/>
    SELECT ?name
    WHERE { ?x foaf:givenName ?name .
        EXISTS { ?x foaf:knows ?who . FILTER(?who != ?x) }
          }


In this example, the pattern is a little more complex. Any graph
pattern is allowed although use of `OPTIONAL` is pointless (which
will always match, possible with no additional results).

`NOT EXISTS` and `EXISTS` can also be used in `FILTER` expressions.
In SPARQL, `FILTER` expressions act over the whole of the basic
graph pattern in which they occur.

    # Names of people who have not stated that they know anyone
    PREFIX foaf: <http://xmlns.com/foaf/0.1/>
    SELECT ?name
    WHERE { ?x foaf:givenName ?name .
        FILTER (NOT EXISTS { ?x foaf:knows ?who })
          }

A note of caution:

    PREFIX foaf: <http://xmlns.com/foaf/0.1/>
    SELECT ?name
    WHERE { ?x foaf:givenName ?name .
        FILTER (NOT EXISTS { ?x foaf:knows ?y })
            ?x foaf:knows ?who
          }

is the same as (it's a single basic graph pattern - the filter does
not break it in two):

    PREFIX foaf: <http://xmlns.com/foaf/0.1/>
    SELECT ?name
    WHERE { ?x foaf:givenName ?name .
            ?x foaf:knows ?y .
        FILTER (NOT EXISTS { ?x foaf:knows ?who })
          }

and the `FILTER` will always be false (`{ ?x foaf:knows ?y }` must
have matched to get to this point in the query and useing ?who
instead makes no difference).


[ARQ documentation index](index.html)