BGP joins

A basic graph pattern (BGP) is a group of triple patterns that must all match. When patterns share a variable, pgRDF joins them on it, and PostgreSQL's planner chooses the join order and indexes.

Examples use the sample data.

Example

People who have both a name and a mailbox:

SELECT * FROM pgrdf.sparql($$
  PREFIX foaf: <http://xmlns.com/foaf/0.1/>
  SELECT ?p ?name ?mbox
  WHERE { ?p foaf:name ?name .
          ?p foaf:mbox ?mbox }
  ORDER BY ?name
$$);
--  {"p": "http://example.org/alice", "mbox": "mailto:alice@example.org", "name": "Alice"}
--  {"p": "http://example.org/carol", "mbox": "mailto:carol@example.org", "name": "Carol"}

?p appears in both patterns, so only subjects that have both properties are returned. Bob has a name but no mailbox, so he is left out; use OPTIONAL to keep him.

Shorthand

Turtle-style abbreviations work in WHERE:

Write	Means
?p foaf:name ?name ; ex:age ?age	; repeats the subject
?p foaf:knows ex:bob , ex:carol	, repeats the subject and predicate
?p a foaf:Person	a is rdf:type

SELECT * FROM pgrdf.sparql($$
  PREFIX foaf: <http://xmlns.com/foaf/0.1/>
  PREFIX ex:   <http://example.org/>
  SELECT ?p WHERE { ?p foaf:knows ex:bob , ex:carol }
$$);
--  {"p": "http://example.org/alice"}

Things to know

• A pattern without a GRAPH clause matches across all graphs, including the default graph. To look in one graph, use GRAPH <iri> { … }.
• Use variables, not blank nodes: _:x or [] in a query pattern is refused with an error.
• A path such as ?s foaf:knows/foaf:name ?n is not supported; write it as two patterns joined on a variable: ?s foaf:knows ?friend . ?friend foaf:name ?n.