# Basic graph queries
The core predicates of the RDF library are rdf/3 and rdf/4, using the signature below.
- rdf(?Subject, ?Predicate, ?Object)
- rdf(?Subject, ?Predicate, ?Object, ?Graph)
If you run the query below by pressing the *play* button SWISH generates a table. The solutions are _rendered_ using the RDF renderer, which renders
- *Resources* in abbreviated form, trying to show the _prefix_, followed
by a label. The label is formed from `rdfs:label`, `skos:prefLabel`,
etc., eventually using the _local name_. By *hovering over the top-left
corner* of the result, the raw Prolog form can be requested, which
is a Prolog atom representing the full IRI.
- *Literals* are represented in their _Turtle_ representation. Again,
by hovering and selecting _Prolog term_, you can view the actual
Clicking a resource will open the ClioPatria _Local View_ on the resource.
rdf(S, P, O).
The rdf/3 and rdf/4 relations are _pure_, i.e., they behave completely logically if part of their arguments is instantiated. So, to get the
`rdf:type` triples, we can use the query below. The _RDF_ prefix is defined using rdf_register_prefix/2, and therefore we can also abbreviate the query:
rdf(S, 'http://www.w3.org/1999/02/22-rdf-syntax-ns#type', O).
rdf(S, rdf:type, O).
## Combining graph queries
Basic rdf/3 patterns can be combined using Prolog's logical connectives.
The basic ones are `A,B` for conjuction (join) and `A;B` for disjunction
(union). SPARQL OPTIONAL is realised by the *->/2 control structure:
`(A *-> true ; true)`, where the first `true` is excuted if `A` has no
solutions and the second for each solution of `A`.
Here is a simple pattern, expressing instances of a direct subclass of
`foaf:Agent`. Note that we need to put single quotes around `Agent` to
turn it into an _atom_; identifiers that start with a capital letter are
rdf(S, rdf:type, C),
rdf(C, rdfs:subClassOf, foaf:'Agent').
## Naming queries
One of the nicest things about using Prolog instead of e.g., SPARQL is that
we can _name_ a pattern. E.g.,
foaf_agent_of_type(Agent, Type) :-
rdf(Agent, rdf:type, Type),
rdf(Type, rdfs:subClassOf, foaf:'Agent').