mirror of
https://github.com/adambard/learnxinyminutes-docs.git
synced 2024-11-29 11:22:14 +03:00
160 lines
6.3 KiB
Markdown
160 lines
6.3 KiB
Markdown
---
|
|
language: RDF
|
|
filename: learnrdf.ttl
|
|
contributors:
|
|
- ["Bob DuCharme", "http://bobdc.com/"]
|
|
---
|
|
|
|
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.
|
|
|
|
```
|
|
# 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
|
|
* [What is RDFS?](https://www.bobdc.com/blog/whatisrdfs/) 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
|
|
|