Sparql

Query Languages: SQL
(SQL Query Language)
 A language for querying collections of tuples:
SELECT SALARY, HIRE_DATE
FROM EMPS
WHERE EMP_ID = 13954

EMP_ID NAME HIRE_DATE SALARY
13954 Joe 2000-04-14 48000
10335 Mary 1998-11-23 52000
… … … …
04182 Bob 2005-02-10 21750

2

Query Languages: XQuery
(XML Query)
 A language for querying trees of XDM nodes:
for $e in document(my_employees.xml)
where $emp/emp/@emp-id = 13954
return $emp/emp/salary

< e m p lo ye e s>

< e m p> < e m p> <e m p> <e m p>

e m p-id = 1 39 54
... ... ...

< n a m e> < h ire -d a te> < s a la ry>

Joe 2 0 00 -04 -14 4 80 00

3

Why an RDF Query Language?
Different XML Representations
 XML at a lower level of abstraction than RDF
 There are various ways of syntactically representing an
RDF statement in XML
 Thus we would require several XQuery queries, e.g.
 //uni:lecturer/uni:title if uni:title element
 //uni:lecturer/@uni:title if uni:title attribute
 Both XML representations equivalent!

4

Two basic families of SQL-like langages for
RDF(S)
 RDQL
 Implementations: Jena, Sesame, RDFStore, ...
 RQL
 Implementations: RQL, SPARQL, ...

5

Introduction to RDQL
 RDF Data Query Language
 JDBC/ODBC friendly

 Simple:

SELECT
some information
FROM
somewhere
WHERE
this match
AND
these constraints
USING
these vocabularies

6

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/>

8

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"

9

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"

10

URI Prefixes : USING
 RDQL has a syntactic convenience that allows prefix strings
to be defined in the USING clause :

SELECT ?x
WHERE (?x, vCard:FN, "John Smith")
USING vCard FOR <http://www.w3.org/2001/vcard-rdf/3.0#>

SELECT ?givenName
WHERE (?y, vCard:Family, "Smith"),
(?y, vCard:Given, ?givenName)
USING vCard FOR <http://www.w3.org/2001/vcard-rdf/3.0#>

11

Filters
 RDQL has a syntactic convenience that allows prefix strings
to be defined in the USING clause :

SELECT ?resource
WHERE (?resource, info:age, ?age)
AND ?age >= 24
USING info FOR <http://somewhere/peopleInfo#>

12

Limitations
 Does not take into account semantics of RDF(S)
 For example:
ex:human rdfs:subClassOf ex:animal
ex:student rdfs:subClassOf ex:human
ex:john rdf:type ex:student

Query: “ To which class does the resource John belong?”
Expected answer: ex:student, ex:human, ex:animal
However, the query:

SELECT ?x
WHERE (<http://example.org/#john>, rdf:type, ?x)
USING rdf FOR <http://www.w3.org/1999/02/22-rdf-syntax-ns#>

Yields only:
<http://example.org/#student>

 Solution: Inference Engines

13

Why an RDF Query Language?
Different XML Representations
 XML at a lower level of abstraction than RDF
 There are various ways of syntactically representing an
RDF statement in XML
 Thus we would require several XPath queries, e.g.
 //uni:lecturer/uni:title if uni:title element
 //uni:lecturer/@uni:title if uni:title attribute
 Both XML representations equivalent!

14

SPARQL
 SPARQL is RDF Query Language
 It provides facilities to:
 extract information in the form of URIs, blank nodes, plain and typed
literals.
 extract RDF subgraphs.
 construct new RDF graphs based on information in the queried graphs

SPARQL Basic Queries
 SPARQL is based on matching graph patterns
 The simplest graph pattern is the triple pattern :
- like an RDF triple, but with the possibility of a variable instead of an RDF
term in the subject, predicate, or object positions
 Combining triple patterns gives a basic graph pattern, where an exact match
to a graph is needed to fulfill a pattern

16

Basic graph pattern
 Set of Triple Patterns
 Triple Pattern – similar to an RDF Triple (subject, predicate, object), but any
component can be a query variable; literal subjects are allowed

?book dc:title ?title

 Matching a triple pattern to a graph: bindings between variables and RDF
Terms

 Matching of Basic Graph Patterns
 A Pattern Solution of Graph Pattern GP on graph G is any substitution S
such that S(GP) is a subgraph of G.
SELECT ?x ?v
WHERE { ?x ?x ?v }
rdf:type rdf:type rdf:Property
x v
rdf:type rdf:Property

Examples
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
SELECT ?c
WHERE
{
?c rdf:type rdfs:Class .
}
 Retrieves all triple patterns, where:
- the property is rdf:type
- the object is rdfs:Class
 Which means that it retrieves all classes

18

Examples (2)
 Get all instances of a particular class (e.g. course) :

PREFIX uni: <http://www.mydomain.org/uni-ns#>
SELECT ?i
WHERE
{
?i rdf:type uni:course .
}

19

Multiple Matches
Data @prefix foaf: <http://xmlns.com/foaf/0.1/> .
_:a foaf:name "Johnny Lee Outlaw" .
_:a foaf:mbox <mailto:jlow@example.com> .
_:b foaf:name "Peter Goodguy" .
_:b foaf:mbox <mailto:peter@example.org> .

PREFIX foaf: <http://xmlns.com/foaf/0.1/> Query
SELECT ?name ?mbox Group Graph Pattern
WHERE (set of graph patterns)
{ ?x foaf:name ?name . also!
?x foaf:mbox ?mbox }

Query Result name mbox
"Johnny Lee <mailto:jlow@example.com>
Outlaw"
"Peter Goodguy" <mailto:peter@example.org>

Using select-from-where
 As in SQL, SPARQL queries have a SELECT-FROM-WHERE structure:
 SELECT specifies the projection: the number and order of retrieved data
 FROM is used to specify the source being queried (optional)
 WHERE imposes constraints on possible solutions in the form of graph
pattern templates and boolean constraints

 Retrieve all phone numbers of staff members:

SELECT ?x ?y
WHERE
{ ?x uni:phone ?y .}

 Here ?x and ?y are variables, and ?x uni:phone ?y represents a resource-
property-value triple pattern

21

Implicit Join
 Retrieve all lecturers and their phone numbers:
SELECT ?x ?y
WHERE
{ ?x rdf:type uni:Lecturer ;
uni:phone ?y . }
 Implicit join: We restrict the second pattern only to those triples, the resource
of which is in the variable ?x
 Notice the syntax shortcut : a semicolon indicates that the following
triple shares its subject with the previous one

22

Implicit join (2)
 The previous query is equivalent to writing:
SELECT ?x ?y
WHERE
{
?x rdf:type uni:Lecturer .
?x uni:phone ?y .
}

23

Explicit Join
 Retrieve the name of all courses taught by the lecturer with ID 949352
SELECT ?n
WHERE
{
?x rdf:type uni:Course ;
uni:isTaughtBy 949352 .
?c uni:name ?n .
FILTER (?c = ?x) .
}

24

Basic Graph Pattern - Blank Nodes

@prefix foaf: <http://xmlns.com/foaf/0.1/> .
Data
_:a foaf:name "Alice" .
_:b foaf:name "Bob" .

PREFIX foaf: <http://xmlns.com/foaf/0.1/> Query
SELECT ?x ?name
WHERE { ?x foaf:name ?name }

x name
Query Result _:c “Alice“
_:d “Bob”

Group Pattern
PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?name ?mbox
WHERE
{ ?x foaf:name ?name .
?x foaf:mbox ?mbox }

SELECT ?name ?mbox
WHERE
{ {?x foaf:name ?name;
foaf:mbox ?mbox }
}

Value Constraints
@prefix dc: <http://purl.org/dc/elements/1.1/> .
Data @prefix : <http://example.org/book/> .
@prefix ns: <http://example.org/ns#> .
:book1 dc:title "SPARQL Tutorial" .
:book1 ns:price 42 .
:book2 dc:title "The Semantic Web" .

PREFIX dc: <http://purl.org/dc/elements/1.1/> Query
PREFIX ns: <http://example.org/ns#>
SELECT ?title ?price
WHERE { ?x ns:price ?price .
FILTER ?price < 30 .
?x dc:title ?title . }
title price
Query Result
"The Semantic Web" 23

Regular expressions can be used
• “Find the name and email addresses of authors of a paper
about SPARQL”

PREFIX dc: <http://purl.org/dc/elements/1.1/>
PREFIX ldap: <http://ldap.hp.com/people#>
PREFIX foaf:

SELECT ?name ?name2
{
WHERE ?doc dc:title ?title .
FILTER regex(?title, “SPARQL”) .
?doc dc:creator ?reseacher .
?researcher ldap:email ?email .
?researcher ldap:name ?name
}

28

Optional graph patterns
<uni:lecturer rdf:about=“949352”>
<uni:name>Grigoris Antoniou</uni:name>
</uni:lecturer>
<uni:professor rdf:about=“94318”>
<uni:name>David Billington</uni:name>
<uni:email>david@work.example.org</uni:email>
</uni:professor>

 For one lecturer it only lists the name
 For the other it also lists the email address

29

Optional graph patterns (2)
 All lecturers and their email addresses:
SELECT ?name ?email
WHERE
uni:name ?name ;
uni:email ?email .
}
 The result of the previous query would be:

?name ?email
David Billington david@work.example.org

 Grigoris Antoniou is listed as a lecturer, but he has no e-mail address

30

Optional graph patterns(3)
 As a solution we can adapt the query to use an optional pattern:

SELECT ?name ?email
WHERE
uni:name ?name .
OPTIONAL { x? uni:email ?email }
}
 The meaning is roughly “give us the names of lecturers, and if known also their e-
mail address”
 The result looks like this:

?name ?email
Grigoris Antoniou
David Billington david@work.example.org

31

Optional graph patterns (4)
@prefix dc: <http://purl.org/dc/elements/1.1/> .
@prefix : <http://example.org/book/> .
Data
@prefix ns: <http://example.org/ns#> .
:book1 dc:title "SPARQL Tutorial" .
:book2 dc:title "The Semantic Web" .

PREFIX dc: <http://purl.org/dc/elements/1.1/>
PREFIX ns: <http://example.org/ns#>
SELECT ?title ?price Query
WHERE { ?x dc:title ?title .
OPTIONAL { ?x ns:price ?price .
FILTER ?price < 30 }} title price
“SPARQL Tutorial“
Query Result
"The Semantic Web" 23

Multiple Optional Blocks
@prefix foaf: <http://xmlns.com/foaf/0.1/> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
Data
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
_:a foaf:name "Alice" .
_:a foaf:homepage <http://work.example.org/alice/> .
_:b foaf:name "Bob" .
_:b foaf:mbox <mailto:bob@work.example> .

SELECT ?name ?mbox ?hpage Query
WHERE { ?x foaf:name ?name .
OPTIONAL { ?x foaf:mbox ?mbox }.
OPTIONAL { ?x foaf:homepage ?hpage } }
Query Result
name Mbox hpage
“Alice“ <http://work.example.org/alice/>
“Bob“ <mailto:bob@example.com>

Alternative Graph Patterns
@prefix dc10: <http://purl.org/dc/elements/1.0/> .
@prefix dc11: <http://purl.org/dc/elements/1.1/> .
Data
_:a dc10:title "SPARQL Query Language Tutorial" .
_:b dc11:title "SPARQL Protocol Tutorial" .
_:c dc10:title "SPARQL" .
_:c dc11:title "SPARQL (updated)" .

PREFIX dc10: <http://purl.org/dc/elements/1.0/>
PREFIX dc11: <http://purl.org/dc/elements/1.1/> Query
SELECT ?x ?y
WHERE { { ?book dc10:title ?x } UNION { ?book dc11:title ?y } } Query Result
x y
"SPARQL (updated)"
"SPARQL Protocol Tutorial"
"SPARQL"
"SPARQL Query Language Tutorial"

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