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diff --git a/rdf.html.markdown b/rdf.html.markdown new file mode 100644 index 00000000..44999022 --- /dev/null +++ b/rdf.html.markdown @@ -0,0 +1,160 @@ +--- +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 + |