1. Logics for Data and Knowledge Representation
SPARQL Protocol and RDF Query Language (SPARQL)
Feroz Farazi 1

2. SPARQL
A language for expressing queries to retrieve information from various data represented in RDF [SPARQL Spec.]
A query language with the capability to search graph patterns [SPARQL Spec.]
Often SPARQL queries contain
a basic graph pattern: a set of subject, object, predicate triple patterns
RDF terms possibly substituted with variables
Result of the query
a subgraph of the RDF data graph

3. Terminologies RDF Terms: RDF Dataset:
Given that I is the set of all IRIs, L is the set of all RDF literals and B is the set of all blank nodes in an RDF graph. Within the graph the set of all RDF Terms, T = I U L U B
RDF Dataset:
D = {G, (I1, G1), (I2, G2),…(Ik, Gk)},
where G is the default graph
(Ii, Gi) are named graphs and i = 1 to k
An RDF dataset always contains a default graph, which does not have a name
It contains zero or more named graphs
Each named graph is identified by a URI

4. Terminologies Triple Pattern: Solution Mapping: Query Variable:
A query variable, v ∈ V, where V is infinite and V ∩ T = ∅
Triple Pattern:
A triple pattern P ∈ {(T U V) x (I U V) x (T U V)}
Solution Mapping:
A solution mapping is a partial function M:V -> T
where V is the query variable and
T is the set of all RDF Terms
Solution Sequence:
A list of solutions which might be unordered. Number of solutions might be zero, one or more.

5. Terminologies Others: IRIs and URIs
Solution Sequence Modifier:
(i) Ordery By (ii) Projection (iii) Distinct (iv) Reduced (v) Offset (vi) Limit
Others:
IRI (Internationalized Resource Identifier), Lexical form, language tag (e.g., en, it), datatype IRI (e.g., xsd:boolean)
IRIs and URIs
URIs include a subset of the ASCII character set
IRIs can include Unicode characters (Universal Character Set)
ASK: to perform a test to know if a query expression has a solution. It replies with yes/no.

6. Query Dataset: Query Expression: Query Result: Dataset:
paper1: :title “Semantic Matching”
Query Expression:
SELECT ?title
WHERE { :paper1 :title ?title. }
Query Result:
“Semantic Matching”
SELECT query form returns RDF Terms bound to the variables
title
Dataset:
paper1: :creator “Fausto Giunchiglia”
Query Expression:
SELECT ?author
WHERE { :paper1 :title ?author. }
Query Result:
“Fausto Giunchiglia”

7. Query Multiple Matches Dataset: Query Expression: Query Result:
_:a :name "Tim Berners-Lee" .
_:a :homepage < .
_:b :name "Fausto Giunchiglia" .
_:b :homepage < .
Query Expression:
SELECT ?name ?homepage
WHERE { ?x :name ?name .
?x :homepage ?homepage }
Query Result:
name homepage
Tim Berners-Lee <
Fausto Giunchiglia <

8. Query RDF Literals Matching Dataset: :x :name "Tim Berners-Lee"@en .
:y :name "Fausto
Query Expression 1:
SELECT ?u
WHERE { ?u :name "Tim Berners-Lee"}
Query Result: u
Query Expression 2:
WHERE { ?u :name "Tim :x
This query has 0 solution because without language tag the search element does not match with dataset element
This query has 1 solution because the inclusion of language tag bound u to :x

9. Building RDF Graphs
CONSTRUCT: this query construct returns an RDF graph
Dataset:
_:a :creator "Tim Berners-Lee" .
_:b :creator "Fausto Giunchiglia" .
Query Expression:
CONSTRUCT { ?x :name ?name }
WHERE { ?x :creator ?name }
Query Result:
_:c :name "Tim Berners-Lee" .
_:d :name "Fausto Giunchiglia" .
In this dataset with :creator we mean Dublin Core (dc) creator metadata
In the query with :name we mean FOAF name metadata
We built a graph with FOAF name attribute which was not available in the source dataset

10. RDF Term Restrictions
FILTER: solutions are restricted to those RDF Terms which match with the filter expression
Dataset:
_:a :creator "Tim Berners-Lee" .
_:a :age 52 .
_:b :creator "Fausto Giunchiglia" .
_:b :age 53.
Query Expression:
SELECT ?author
WHERE { ?x :creator ?author.
FILTER regex(?author, "Tim") }
Query Result:
author
"Tim Berners-Lee" .
The above query can be made case insensitive by adding “i” flag in the filter as follows:
FILTER regex(?author, “tim”, “i”)
Query Expression:
SELECT ?author ?age
WHERE { ?x :creator ?author.
?x :age ?age
FILTER (?age >52) }
Query Result:
author age
"Fausto Giunchiglia" 53

11. Querying Optional Pattern
OPTIONAL: to allow binding variables to RDF Terms to be included in the solution in case of availability
Dataset:
_:a :creator "Tim Berners-Lee" .
_:a :age 52 .
_:a :homepage < .
_:b :creator "Fausto Giunchiglia" .
_:b :age 53.
Query Expression:
SELECT ?author ?homepage
WHERE { ?x :creator ?author.
OPTIONAL {?x :homepage ?homepage}}
Query Result:
author homepage
"Tim Berners-Lee" <
"Fausto Giunchiglia"
It is a left associative operator
Why do we need it? All entities might not have the same set of attributes

12. ORDER BY Clause ORDER BY: a facility to order a solution sequence
Dataset:
_:a :creator "Tim Berners-Lee" .
_:a :age 52 .
_:b :creator "Fausto Giunchiglia" .
_:b :age 53.
Query Expression:
SELECT ?author
WHERE { ?x :creator ?author;
?x :age ?age}
ORDER BY ?author DESC (?age)
Query Result:
author
"Fausto Giunchiglia"
"Tim Berners-Lee"

13. DISTINCT and REDUCED Modifiers
DISTINCT: to remove duplicate from a solution sequence
Dataset:
_:b :creator "Fausto Giunchiglia" .
_:b :age 53.
_:c :creator "Fausto Giunchiglia" .
_:c :age 37.
Query Expression:
SELECT DISTINCT ?creator
WHERE { ?x :creator ?creator}
Query Result:
creator
"Fausto Giunchiglia"
REDUCED: to permit the duplicates to be removed.
SELECT REDUCED ?creator
The cardinality of the elements in the solution set is at least one and no more than the cardinality without removing duplicates

14. OFFSET and LIMIT Clauses
OFFSET: to show the elements of the solution set starting after a specified number. If the number is zero, there will be no effect.
Dataset:
_:b :creator "Fausto Giunchiglia" .
_:b :age 53.
_:c :creator "Tim Berners-Lee" .
_:c :age 52.
Query Expression:
SELECT ?author
WHERE { ?x :creator ?author }
ORDER BY ?author
OFFSET 1
Query Result:
author
"Tim Berners-Lee"
Limit: to put an upper bound on the number of elements of the solution set returned
Query Expression:
SELECT ?author
WHERE { ?x :creator ?author }
ORDER BY ?author
LIMIT 1
OFFSET 1
Query Result:
author
"Tim Berners-Lee"

15. Relational vs RDF queries
Relational queries consist of (among others):
Relational algebra of joins
Foreign key references
RDF queries consists of (among others):
(Logical) statements in triple form
Unification variables are used to connect graph patterns
A relational query:
Produces a new database table that is a combination of two or more input tables (partially or completely)
An RDF query:
Produces a subset of the input RDF graph
Simplifies some issues of table based queries, for example, no need to put subquery construct
[D. Allemang and J. Hendler, 2008]

16. References SPARQL Spec. (2008). W3C Recommendation.
D. Allemang and J. Hendler. Semantic web for the working ontologist: modeling in RDF, RDFS and OWL. Morgan Kaufmann Elsevier, Amsterdam, NL, 2008.
R. de Virgilio, F. Giunchiglia and Letizia Tanca. Semantic Web Information management, a model based perspective. Springer, 2009.