3.2.1 Query the RDF database

[nondet]rdf(?Subject, ?Predicate, ?Object)

Elementary query for triples. Subject and Predicate are atoms representing the fully qualified URL of the resource. Object is either an atom representing a resource or literal(Value) if the object is a literal value. If a value of the form NameSpaceID:LocalName is provided it is expanded to a ground atom using expand_goal/2. This implies you can use this construct in compiled code without paying a performance penalty. Literal values take one of the following forms:

Atom

If the value is a simple atom it is the textual representation of a string literal without explicit type or language qualifier.

lang(LangID, Atom)

Atom represents the text of a string literal qualified with the given language.

type(TypeID, Value)

Used for attributes qualified using the rdf:datatype TypeID. The Value is either the textual representation or a natural Prolog representation. See the option convert_typed_literal(:Convertor) of the parser. The storage layer provides efficient handling of atoms, integers (64-bit) and floats (native C-doubles). All other data is represented as a Prolog record.

For literal querying purposes, Object can be of the form literal(+Query, -Value), where Query is one of the terms below. If the Query takes a literal argument and the value has a numeric type numerical comparison is performed.

plain(+Text)

Perform exact match and demand the language or type qualifiers to match. This query is fully indexed.

icase(+Text)

Perform a full but case-insensitive match. This query is fully indexed.

exact(+Text)

Same as icase(Text). Backward compatibility.

substring(+Text)

Match any literal that contains Text as a case-insensitive substring. The query is not indexed on Object.

word(+Text)

Match any literal that contains Text delimited by a non alpha-numeric character, the start or end of the string. The query is not indexed on Object.

prefix(+Text)

Match any literal that starts with Text. This call is intended for completion. The query is indexed using the skip list of literals.

ge(+Literal)

Match any literal that is equal or larger then Literal in the ordered set of literals.

gt(+Literal)

Match any literal that is larger then Literal in the ordered set of literals.

eq(+Literal)

Match any literal that is equal to Literal in the ordered set of literals.

le(+Literal)

Match any literal that is equal or smaller then Literal in the ordered set of literals.

lt(+Literal)

Match any literal that is smaller then Literal in the ordered set of literals.

between(+Literal1, +Literal2)

Match any literal that is between Literal1 and Literal2 in the ordered set of literals. This may include both Literal1 and Literal2.

like(+Pattern)

Match any literal that matches Pattern case insensitively, where the `*' character in Pattern matches zero or more characters.

Backtracking never returns duplicate triples. Duplicates can be retrieved using rdf/4. The predicate rdf/3 raises a type-error if called with improper arguments. If rdf/3 is called with a term literal(_) as Subject or Predicate object it fails silently. This allows for graph matching goals like rdf(S,P,O),rdf(O,P2,O2) to proceed without errors.

[nondet]rdf(?Subject, ?Predicate, ?Object, ?Source)

As rdf/3 but in addition query the graph to which the triple belongs. Unlike rdf/3, this predicate does not remove duplicates from the result set.

Source

is a term Graph:Line. If Source is instatiated, passing an atom is the same as passing Atom:_.

[nondet]rdf_has(?Subject, +Predicate, ?Object)

Succeeds if the triple rdf(Subject, Predicate, Object) is true exploiting the rdfs:subPropertyOf predicate as well as inverse predicates declared using rdf_set_predicate/2 with the inverse_of property.

[nondet]rdf_has(?Subject, +Predicate, ?Object, -RealPredicate)

Same as rdf_has/3, but RealPredicate is unified to the actual predicate that makes this relation true. RealPredicate must be Predicate or an rdfs:subPropertyOf Predicate. If an inverse match is found, RealPredicate is the term inverse_of(Pred).

[nondet]rdf_reachable(?Subject, +Predicate, ?Object)

Is true if Object can be reached from Subject following the transitive predicate Predicate or a sub-property thereof, while repecting the symetric(true) or inverse_of(P2) properties.

If used with either Subject or Object unbound, it first returns the origin, followed by the reachable nodes in breath-first search-order. The implementation internally looks one solution ahead and succeeds deterministically on the last solution. This predicate never generates the same node twice and is robust against cycles in the transitive relation.

With all arguments instantiated, it succeeds deterministically if a path can be found from Subject to Object. Searching starts at Subject, assuming the branching factor is normally lower. A call with both Subject and Object unbound raises an instantiation error. The following example generates all subclasses of rdfs:Resource:

?- rdf_reachable(X, rdfs:subClassOf, rdfs:'Resource').
X = 'http://www.w3.org/2000/01/rdf-schema#Resource' ;
X = 'http://www.w3.org/2000/01/rdf-schema#Class' ;
X = 'http://www.w3.org/1999/02/22-rdf-syntax-ns#Property' ;
...

[nondet]rdf_reachable(?Subject, +Predicate, ?Object, +MaxD, -D)

Same as rdf_reachable/3, but in addition, MaxD limits the number of edges expanded and D is unified with the `distance' between Subject and Object. Distance 0 means Subject and Object are the same resource. MaxD can be the constant infinite to impose no distance-limit.