Applications can use the ASK form to test whether or not a query pattern has a solution. No information is returned about the possible query solutions, just whether or not a solution exists.
@prefix foaf: <http://xmlns.com/foaf/0.1/> .\n\n_:a foaf:name \"Alice\" .\n_:a foaf:homepage <http://work.example.org/alice/> .\n\n_:b foaf:name \"Bob\" .\n_:b foaf:mbox <mailto:bob@work.example> .\nPREFIX foaf: <http://xmlns.com/foaf/0.1/>\nASK { ?x foaf:name \"Alice\" }\ntrue\nThe SPARQL Query Results XML Format form of this result set gives:
\n<?xml version=\"1.0\"?>\n<sparql xmlns=\"http://www.w3.org/2005/sparql-results#\">\n <head></head>\n <boolean>true</boolean>\n</sparql>\nOn the same data, the following returns no match because Alice's mbox is not mentioned.
PREFIX foaf: <http://xmlns.com/foaf/0.1/>\nASK { ?x foaf:name \"Alice\" ;\n foaf:mbox <mailto:alice@work.example> }\nfalse\n