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Querying the Semantic Web
*
with RQL
G. Karvounarakis, A. Magganaraki S. Alexaki , V. Christophides , D.
Plexousakis, M. Scholl , K. Tolle , The International Journal of Computer and
Telecommunications Networking , Volume 42, 617 - 640 , 2003
DBLab 고명석
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Contents
Introduction
Motivating Example
A Formal Model for RDF
The RDF Query Language : RQL
Conclusion
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Introduction
Next evolution step of the web
Vast amount of information
Various kinds of descriptive information
Better knowledge
The meaning, usage, accessibility or quality of
web resource
RDF enable the creation and exchange of
resource metadata
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Introduction
RDF Provides
Standard Representation Language
For metadata based on directed labeled graph
Schema Definition Language
For creating vocabularies of labels for these graph
node
XML syntax for expressing metadata and
schemas
Humanly readable and machine understandable
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Motivating Example
String
fname
exhibited
creates
String
Museum
Artifact
Artist
String
lname
String
material
sculpts
Sculptor
Sculpture
String
mime-type
title
String
file-size
paints
Painter
“Pablo”
Painting
Ext. Resource
String
exhibited
fname
“Gogh”
“Rodin”
lname
exhibited
fname
creates
&r6
“August”
&r5
lname
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subclassOf (isA)
subPropertyof
&r8
title
exhibited
&r7
mime-type
“Reina Sofia
Museum”
typeOf (instance)
“oil on
Canvas”
technique
paints
last_modified
“oil on
Canvas”
&r2
lname
&r4
&r3
paints
technique
“Van”
String
last_modified
title
fname
&r1
“Picaso”
technique
“image/jpeg”
last_modified
“Rodin
Museum”
2000-02-01
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The RDF Query Language : RQL
RQL Provides
Select-form-where filter
Standard Boolean matching
=, <, >, like
Ex) “Painter” < “Artist”
{ }
To introduce appropriate schema or data variables
“^”
Only the node labeled with the class name
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Example
Q1 : Which classes can appear as domain
and range of the property creates?
$C1
$C2
Artist
Artifact
Artist
Painting
Artist
Sculpture
Painter
Artifact
Painter
Painting
Painter
Sculpture
Sculptor
Artifact
Sculptor
Painting
Sculptor
Sculpture
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Select $C1, $C2 from {$C1} creates {$C2}
“C1 <= domain(creates) and
C2 <= range(creates)”
Subclassof(domain(creates)) and
Subclassof(range(creates))
• Class variable are prefixed by $
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Example
Q2 : Find all properties(and their range) that
are applicable on class Painter
@P
Range(@P)
creates
Artifact
paints
Painting
lname
string
fname
string
Select @P, range(@P)
from {$C}@P
Where $C = Painter
domain(P) >= Painter
 @P is property variable
{x;C}
Filters data node x which are labeled with a class name
{;C}
Simply denotes a filtering condition of schema nodes
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Example
Q3: Fine all information related to class
Painter(i.e., its superclasses as well as direct or
inherited properties)
Seq(Painter, superclassof^(Painter),
(select @P, domain(@P), range(@P)
from {;Painter}@P))
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Example
Q4: What properties can be reached(in one
step) from the range classes of creates?
$Y
@P
range(@P)
Artifact
exhibited
Museum
Painting
exhibited
Museum
Sculpture
exhibited
Museum
Painting
technique
string
Sculpture
material
string
Select $Y, @P, range(@P)
from creates{$Y}.@P
$Y <= domain(@P) and
$Y <= range(creates)
“ . ”
Notation implies a join condition between the range
classes of the property creates and the domain of @P
valuations
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Example
Q5: Fine the Museum resources that have
been modified after year 2000
Select X, Y
Form Museum{X}.last_modified{Y}
Where Y >= 2000-01-01
Return to site www.museum.es(&r4)
Return to site www.rodin.fr(&r7)
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Example
Q6:Find the names of Artists whose Artifacts
are exhibited in museums, along with the
related Museum title
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Example
Select V, R, Y, Z
From {X}creates.exhibited{Y}.title{Z},
{X}fname{V}, {X}lname{R}
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Example
Q7: Fine the source and target value of
properties emanating from ExtResources
X
Y
&r6
“image/jpg”
&r7
“Rodin Museum”
&r4
“Reina Sofia Museum”
&r7
2000-06-09
&r4
2000-02-01
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Select X, Y
from {X; ExtResource}@P{Y}
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Example
Q8: Find the descriptions of resources whose
URI matches “www.museum.es”
Select X, (select $W, (select @P, Y from {X;$W}@P{Y})
from $W{X})
From Resource{X}
Where X like “www.museum.es”
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Example
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Example
Q9: Find the description, under the form of
triples, of resources excluding properties
related to the class ExtResource
((Select X, @P, Y from {X}@P{Y})
Union
(Select X, type, $W from $W{X}))
minus
((Select X, @P, Y from {X;ExtResource}@P{Y})
union
(Select X, type, ExtResorse from ExtResorse{X}))
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RDF Query Language
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SquishQL
Use SQL-like constructs to reflect RDF‘
graph syntax
Variables are indicated by ‘?’
Most general pattern is (?x, ?y, ?z)
There are two classes of constraints
Patterns
<subject, predicate, object> describing edges of the gr
aph and conjunction operator
Filter expressions
Remove the possibility
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SquishQL
Query typically built on five clauses
SELECT Clause
Identifies the variables to be returned to the
application
FROM Clause
specifies the model by URI
WHERE Clause
Specifies the graph pattern as the conjunction of the
list of triple patterns
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SquishQL
Query typically built on five clauses
AND Clause
Specifies the Boolean expressions over values of
URIs and literals
arithmetic comparisons, and boolean expressions,
including disjunction and negation
USING Clause
A way to shorten the length of URIs
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SquishQL - Example
Query: Finds the titles of documents in
http://example.com/xmleurope/presentations.rdf
SELECT ?title
FROM http://example.com/xmleurope/presentations.rdf
WHERE
(?doc, <dc:title>, ?title),
(?doc, <rdf:type>, <foaf:Document>)
USING
dc FOR <http://purl.org/dc/elements/1.1/>,
rdf FOR <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
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RDFPath
Plan to make an XPath*-like query system
for the RDF data model
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RDFPath
Node Queries
/
Returns all nodes.
[ ]
Returns all anonymous nodes
[foaf:mbox "[email protected]"]
Returns all nodes where the value of the property foaf:mbox
equals the literal [email protected]
[foaf:mbox "[email protected]" ; foaf:homepage
<http://www.aaronsw.com>]
Returns nodes where foaf:mbox equals the literal of my email
address and foaf:homepage equals the resource of my homepage
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RDFPath
Node Queries
[a rdf:Bag]
Returns nodes of type rdf:Bag
<http://example.org/#foo>
Returns the resource <http://example.org/#foo>
"Whatever"
Returns the literal "Whatever"
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RDFPath
Sub-queries
/dc:creator
Returns values of the dc:creator property for all
resources
/[a rdfs:ContainerMembershipProperty]
Returns the values of all container membership
properties (_1, _2, etc.) for all resources
"Aaron Swartz"\dc:creator
Returns all resources with a dc:creator property of
"Aaron Swartz"
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RDFPath
Query Refinements
/dc:creator [a w3c:StaffMember]
Returns all of the w3c:StaffMembers who are listed as
the dc:creator of a resource
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TRIPLE
Goal
use various semantics in one inference
 Information integration
Syntactically inspired by F-Logic:
subject[predicateobject]
Example
Sara [livesIn → Linköping; hasAge → 35]
Sara [hasChild → Lotta[hasAge → 5]]
Sara[believes →<Lotta[hasIllness → cold]>]
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TRIPLE
model1
model2
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Set of Rules
Describing
Semantics of
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TRIPLE – language description
Namespace and resource abbreviations:
rdf := “http://www.w3.org/1999/02/22-rdf-syntax-ns#”
isa := rdf:subClassOf
Statements, triples, molecules:
subject[predicateobject]
subject[p1o1; p2 o2; ...]
s1[p1  s2[p2o] ]
Models, model expressions, parameterized models:
s[po]@m
“triple <s,p,o> in model m”
s[po]@(m1  m2)
model intersection, union, diff.
s[po]@sf(m1, X, Y) Skolem function
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TRIPLE - Example
namespace abbreviations
dc := “http://purl.org/dc/elements/1.0/”.
dfki := “http://www.dfki.de/”.
block
@dfki:documents {
dfki:d_01_01 [
dc:title  “TRIPLE”;
fact
TRIPLE
dc:title
dc:creator  “Michael Sintek”;
dc:subject  RDF;
dc:subject  triples; ... ].
Michael Sintek
dc:creator
dfki:d_01_01
dc:subject
RDF
dc:subject
...
triples
}
Query: FORALL D <- D[dc:subject  RDF]@dfki:documents
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TRIPLE - Example
Cars Ontology with RDF Schema Semantics
@cars {
xyz:MotorVehicle
xyz:MotorVehicle[rdfs:subClassOf -> rdfs:Resource].
xyz:PassengerVehicle[rdfs:subClassOf -> xyz:MotorVehicle].
xyz:Truck
xyz:Van
xyz:Truck[rdfs:subClassOf -> xyz:MotorVehicle].
xyz:Van[rdfs:subClassOf -> xyz:MotorVehicle].
xyz:PassengerVehicle
xyz:MiniVan[
rdfs:subClassOf -> xyz:Van;
xyz:MiniVan
rdfs:subClassOf -> xyz:PassengerVehicle].
}
X = xyz:Van
FORALL X <X[rdfs:subClassOf -> xyz:MotorVehicle]@cars.
FORALL X <X[rdfs:subClassOf -> xyz:MotorVehicle]@rdfschema(cars).
X = xyz:Truck
X = xyz:PassengerVehicle
X = xyz:Van
X = xyz:Truck
X = xyz:PassengerVehicle
X = xyz:MiniVan
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VERSA
Graphic-based query language for querying
RDF Model
the traversal expression which matches
patterns in the model's graph
all() - dc:date -> *
all() |- rdfs:label -> eq("Web [ 1 ]")
<List>
<String>2001-03-04</String>
<String>2001-04-20</String>
</List>
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RDFQL
RDFQL is the language used by RDF
Gateway applications and agents
RDF Gateway
A platform for Semantic Web applications and agents
A simple SQL-style language
with inference extensions to perform complex
deductive queries
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RDFQL
SELECT ?b
FROM <http://weblog.burningbird.net/resume.rdf>
WHERE (?a, <bbd:title>, ?b)
USING bbd for >http://www.burningbird.net/resume_schema#>
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Comparison of Query language
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Comparison of Query language
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Comparison of Query language
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Comparison of Query language
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