Merge pull request #4486 from bobdc/add-rdf

[rdf/en] Add page for RDF (Resource Description Framework)

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