Ontology Query
What is an Ontology
• Ontologies resemble faceted taxonomies but use richer semantic
relationships among terms and attributes, as well as strict rules
about how to specify terms and relationships. Because ontologies
do more than just control a vocabulary, they are thought of as
knowledge representation. The often-quoted definition of ontology is
"the specification of one's conceptualization of a knowledge
domain." (~Tom Gruber)
– iawiki.net/IAGlossary
Some Related Languages For Web
Ontology
• XML provides a surface syntax for structured documents, but
imposes no semantic constraints on the meaning of these
documents.
• XML Schema is a language for restricting the structure of XML
documents and also extends XML with data types.
• RDF is a data model for objects ("resources") and relations between
them, provides a simple semantics for this data model, and these
data models can be represented in an XML syntax.
• RDF Schema is a vocabulary for describing properties and classes
of RDF resources, with a semantics for generalization-hierarchies of
such properties and classes.
• OWL adds more vocabulary for describing properties and classes:
among others, relations between classes (e.g. disjointness),
cardinality (e.g. "exactly one"), equality, richer typing of properties,
characteristics of properties (e.g. symmetry), and enumerated
classes.
Ontology Related Language
language
Description
Query Language
XML
Structured Documents
XQuery, XPath
data model for objects
RDQL, RQL, Versa, Squish
data model + Relations
OWL-QL, Jena
Data model + relations + rules
(owl + RuleML )
??
Extensible Markup Language
RDF
Resource Description Framework
OWL
Web Ontology Language
SWRL
Semantic Web Rule Language
RDQL
•
•
•
•
RDF Data Query Language.
is a query language for RDF.
is declarative.
Considers RDF model as a set of triples:
– (Subject Property Value)
• Permits to specify patterns that match
versus triples of the model to return a
result.
RDQL: RDF Data Query
Language
• Basic Syntax:
SELECT vars
FROM documents
WHERE Expressions
AND Filters
USING Namespace declarations
Clauses
• SELECT Clause
– Identifies the variables to be returned to the
application. If not all the variables are needed by the
application, then specifying the required results can
reduce the amount of memory needed for the results
set as well as providing information to a query
optimizer.
• FROM Clause
– The FROM clause specifies the model by URI.
• WHERE Clause
– This specifies the graph pattern as a list of triple
patterns.
Clauses
• AND Clause
– Specifies the Boolean expressions,
– indicates constraints that RDQL variables must follow
• USING Clause
– A way to shorten the length of URIs. This mechanism
helps make for an easier to understand syntax. This
is not a namespace mechanism; instead it is simple
an abbreviation mechanism for long URIs by defining
a string prefix.
Clauses in-depth: SELECT
• The SELECT portion of the query let's you
indiate which RDQL variables you want to
be returned by the query, if you use
SELECT ?x,?y,?z then you will receive an
array of tuples containing values for ?x,?y
and ?z. You can use other variables in the
query such as ?a1,?p,?foo but they won't
be returned since they are not present in
the select part of the query.
Clauses in-depth: FROM
• The FROM part of the query indicates the
RDF sources to be queried, each source is
enclosed by angle brackets (<&>). If you
indicate more than one source sepparate
them using commas.
Clauses in-depth: WHERE
• The where part is the most important part
of the RDQL expression, in the where part
you indicate constraints that RDF triples
(subject, predicate, object) must
accomplish in order to be returned. The
where part is expressed by a list of
restrictions separated by commas, each
restriction takes the form: (subject,
predicate, object) where the subject,
predicate and object can be a literal value
or a RDQL variable.
Clauses in-depth: WHERE
• For the predicate you can express
property names using a namespace
declared in the USING section for
example: <dc:name> which indicates that
the predicate must match the "name"
local-name for the namespace declared as
"dc" in the using part.
Clauses in-depth: AND
• The AND part indicates constraints that
RDQL variables must follow. In the PHP
implementation the AND part is a PHP
expression where variables are RDQL
variables such as ?x,?y etc.
Clauses in-depth: USING
• The USING section declares all the
namespaces that will be used for RDF
properties, declarations are sepparated by
commas and use the notation:
Syntax: Where, And
• Where: indicate constraints that RDF
triples (subject, predicate, object)
• And: indicates constraints that RDQL
variables must follow
Example
Example
• q1 contains a query:
SELECT ?x
WHERE (?x <http://www.w3.org/2001/vcard-rdf/3.0#FN> "John Smith")
• For executing q1with a model m1.rdf:
java jena.rdfquery --data m1.rdf --query q1
• The outcome is:
x
=============================
<http://somewhere/JohnSmith/>
Example
• Return all the resources that have property FN
and the associated values:
SELECT ?x, ?fname
WHERE (?x <http://www.w3.org/2001/vcard-rdf/3.0#FN> ?fname)
• The outcome is:
x
| fname
================================================
<http://somewhere/JohnSmith/>
| "John Smith"
<http://somewhere/SarahJones/>
| "Sarah Jones"
<http://somewhere/MattJones/>
| "Matt Jones"
Example
• Return the first name of Jones:
SELECT ?givenName
WHERE (?y <http://www.w3.org/2001/vcard-rdf/3.0#Family> "Jones"),
(?y <http://www.w3.org/2001/vcard-rdf/3.0#Given> ?givenName)
• The outcome is:
givenName
=========
"Matthew"
"Sarah"
Other Examples
• Indicate name and age of all the
individuals older than 20
SELECT ?y
FROM <people.rdf>
WHERE (?x,<dt:age>,?z),(?x,<dt:name>,?y)
AND ?z>20
USING dt for <http://foo.org#>, rdf for <http://www.w3.org/1999/02/22rdf-syntax-ns#>
RQL - cont
• Class & Property Querying
– Which classes can appear as domain and range of
the property creates?
• SELECT $C1, $C2
• FROM {$C1}creates{$C2}
• SELECT X, Y
• FROM Class{X}, Class{Y}, {;X}creates{;Y}
– What if?
• SELECT X, Y
• FROM {X}creates{Y}
RQL - cont
• Querying Resource Descriptions
• SELECT X
• FROM Artist{X}
• SELECT X, Y
• FROM {X}creates{Y}
RQL - cont
• SELECT Y
• FROM {X}title{Y}
• WHERE X like "*www.artchive.com*“
Using from Java Code
• It is possible to run RDQL queries from
the Java application.
• The following classes are to be used for
this:
– Query
– QueryExecution
– QueryEngine
– QueryResults
– ResultBinding
Example
SELECT ?x, ?fname
WHERE (?x <http://www.w3.org/2001/vcard-rdf/3.0#FN> ?fname)
Query query = new Query("SELECT...") ;
query.setSource(model);
QueryExecution qe = new QueryEngine(query) ;
QueryResults results = qe.exec();
for (Iterator iter = results; iter.hasNext();)
{
ResultBinding res = (ResultBinding) iter.next();
Resource x = (Resource) res.get("x");
Literal fname = (Literal) res.get("fname");
System.out.println("x: " + x + " fname: " + fname);
}
Persistent Models
• Jena permits to create persistent models:
– such as with relational databases.
• Jena 2 supports:
– MySQL
– Oracle
– PostgreSQL
• To create a persistent model:
– ModelFactory.createModelRDBMaker(conn).createModel()
Example
// Create a connection to DB
DBConnection c = new DBConnection(DB_URL, DB_USER, DB_PASS, DB_TYPE);
// Create a ModelMaker for persistent models
ModelMaker maker = ModelFactory.createModelRDBMaker(c);
// Create a new model
Model model = maker.createModel("modelo_1");
// Start transaction
model.begin();
// Read a model from an XML archive
model.read(in, null);
// Commit a transaction
model.commit();