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Merge pull request #4486 from bobdc/add-rdf
[rdf/en] Add page for RDF (Resource Description Framework)
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Marcel Ribeiro Dantas
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---
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language: RDF
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filename: learnrdf.ttl
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contributors:
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- ["Bob DuCharme", "http://bobdc.com/"]
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lang: en-en
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---
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RDF (Resource Description Framework) is a [W3C
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standard](https://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/) data
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model. The W3C has standardized several RDF syntaxes; examples below use the
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most popular one, [Turtle](https://www.w3.org/TR/turtle/).
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One nice advantage of Turtle files is that if you concatenate any two
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syntactically valid Turtle files, you will have another syntactically valid
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Turtle file. This is one of many things about RDF that ease data integration.
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The W3C standard query language for RDF datasets is
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[SPARQL](https://www.w3.org/TR/sparql11-query/).
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RDF expresses all facts as three-part {subject, predicate, object} statements
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known as triples. Because the same entity can be the subject of some triples
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and the object of others, a set of triples can represent a graph data
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structure. A large-scale storage system for triples is called a triplestore,
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and falls into the graph database category of NoSQL databases.
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RDF subjects and predicates must be URIs (Uniform Resource Identifiers), which
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usually look like URLs but function as identifiers, not locators. The use of
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URIs provides context for resource identifiers to make them unambiguous—for
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example, to tell a book title from a job title.
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```turtle
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# The hash symbol is the comment delimiter.
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# Turtle triple statements end with periods like natural language sentences.
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# These two triples tell us that the mythical Example Company's
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# employee 134 has a hire date of 2022-11-12 and a family name of Smith:
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<http://example.com/emp134> <http://example.com/hireDate> "2022-11-12" .
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<http://example.com/emp134> <http://example.com/familyName> "Smith" .
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# Declaring prefixes to stand in for namespaces reduces verbosity. These
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# declarations typically go at the beginning of the file, but the only
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# requirement is that they come before the first use of the prefix they declare.
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@prefix ex: <http://example.com/> .
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ex:emp134 ex:hireDate "2022-11-12" .
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ex:emp134 ex:familyName "Smith" .
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# A semicolon means that the next triple uses the same subject as the last
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# one. This is handy for listing data about a single resource. The following
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# example means the same thing as the previous one.
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@prefix ex: <http://example.com/> .
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ex:emp134 ex:hireDate "2022-11-12" ;
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ex:familyName "Smith" .
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# A comma means that the next triple has the same subject and predicate as
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# the previous one.
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ex:emp134 ex:nickname "Smithy", "Skipper", "Big J".
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# Three single or double quote marks at the beginning and end of a value let
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# you define a multi-line string value.
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ex:emp134 ex:description """
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Skipper joined the company in November.
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He always has a joke for everyone.""" .
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# Using URIs from existing standard vocabulary namespaces eases both data
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# integration and interoperability with the large amount of RDF that already
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# exists. Mixing and matching of standard and local custom namespaces is
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# common.
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@prefix vcard: <http://www.w3.org/2006/vcard/ns#> .
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ex:emp134 ex:hireDate "2022-11-12" ;
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vcard:family-name "Smith" .
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# Related RDF standards provide vocabularies that are popular for basic
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# facts. The rdfs:label predicate from the RDF Schema standard is a common
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# way to indicate a human-readable name.
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@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
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ex:hireDate rdfs:label "hire date" .
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# String object values can include language codes, making
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# multi-lingual representation of entities easier for applications
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# reading the data (for example, when generating a user interface).
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ex:hireDate rdfs:label "hire date"@en, "date d'embauche"@fr .
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# Representing a triple's object with a URI (or prefixed name) is not required
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# but lets you connect up triples into a graph.
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ex:emp134 vcard:family-name "Smith" .
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ex:emp113 vcard:family-name "Jones" ;
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ex:reportsTo ex:emp134 .
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# Objects can be datatypes from the XML Schema part 2 standard or your own
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# custom datatypes.
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@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
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ex:emp134 vcard:family-name "Smith"^^xsd:string ; # default data type
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ex:hireDate "2022-11-12"^^xsd:date ;
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ex:rating "3.5"^^ex:someCustomType .
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# The use of schemas with RDF is optional. Schemas may describe all or a
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# subset of a dataset. They use a vocabulary described by the W3C RDF Schema
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# (RDFS) standard, usually with a prefix of rdfs.
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# These schemas are descriptive, to ease the accommodation of new
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# datasets, not proscriptive rules about how new data should be
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# created. The following declares a class. (Note that RDFS is itself
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# expressed in triples.)
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@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
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ex:Person rdf:type rdfs:Class .
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# The following triple means the same as the preceding one but
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# uses a Turtle shortcut for terseness and more readability.
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ex:Person a rdfs:Class .
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# That last triple declares that ex:Person is an instance of a class, and the
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# following declares that employee 113 is an instance of the class Employee.
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ex:emp113 a ex:Employee .
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# The first triple below is actually unnecessary because a typical
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# RDFS processor will infer from the second one that ex:Employee is a
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# class. (Only a subset of RDF parsers perform RDFS inferencing.)
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ex:Employee a rdfs:Class .
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ex:Employee rdfs:subClassOf ex:Person .
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# An RDF parser that reads the last four triples shown and understands
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# RDFS will infer that ex:emp113 is an instance of ex:Person, because
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# it's an instance of ex:Employee, a subclass of ex:Person.
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# RDFS lets you declare properties and associate them with classes.
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# Properties are first class resources and don't "belong" to classes
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# in the object-oriented sense. rdfs:domain means "the following object
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# class uses the property named by this triple's subject". rdfs:range
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# means "the property named by this triple's subject will have a value of
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# the following class or type".
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ex:birthday rdf:type rdf:Property ;
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rdfs:domain ex:Person ;
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rdfs:range xsd:date .
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```
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## Further Reading
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* [RDF Primer — Turtle version](https://www.w3.org/2007/02/turtle/primer/) from the W3C
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* [What is RDF?](https://www.bobdc.com/blog/whatisrdf/) on bobdc.com
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* [Introduction to RDF and SPARQL](https://data.europa.eu/sites/default/files/d2.1.2_training_module_1.3_introduction_to_rdf_sparql_en_edp.pdf) at data.europa.eu
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