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, i = 1 to k, k ≥ 0
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) Order 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), literal, plain literal and typed literal
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 :creator ?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. Turtle Turtle is a terse RDF triple language
Turtle is a textual syntax for RDF facilitates writing RDF graph
in a compact and natural language text form
with abbreviations for common usage patterns and datatypes
compatible with triple pattern syntax of SPARQL (and N-Triples)
Triple:
a sequence of (subject, predicate, object) terms
separated by whitespace
terminated by '.' after each triple
e.g., < < < .
List of predicates and objects:
For the same subject can be codified without repeating the common part
Reference to the subject of the previous triple is indicated by the use of a semicolon
e.g., < < < ;
< "Weaving the Web".

17. Turtle List of objects: SPARQL differs from Turtle:
For the same subject and predicate can be codified without repeating the common part
Reference to the subject and predicate of the previous triple is indicated by the use of a comma
e.g., < < "Tim Berners-Lee", "TBL", "Tim BL" .
SPARQL differs from Turtle:
SPARQL permits RDF Literals as the subject of RDF triples
SPARQL permits variables (?name or $name) in any part of the triple of the form
prefix and base declarations
Turtle allows prefix and base declarations anywhere outside of a triple
In SPARQL, they are only allowed in the Prologue (at the start of the SPARQL query)
case sensitivity
SPARQL uses case insensitive keywords, except for 'a‘, where 'a' means the IRI
Turtle's prefix and base declarations are case sensitive
'true' and 'false' are case insensitive in SPARQL and case sensitive in Turtle
TrUe is not a valid boolean value in Turtle

18. SPARQL: Federated Query
Federated SPARQL query can be used to express queries across diverse data sources if
data is stored natively as RDF or
data is viewed as RDF via middleware
It is an opportunity for data consumers to get data distributed across the Web
Federated Query is used for executing queries distributed over different SPARQL endpoints
The SERVICE keyword
allows a query author to direct a portion of a query to a particular SPARQL endpoint
supports SPARQL queries merging data distributed across the Web

19. SPARQL: Federated Query
An example query to a remote SPARQL endpoint
Consider a query to find the names of the people we know
Data about the names of various people is available at the endpoint:
and one wants to combine with a local FOAF file that contains the single triple:
DATASET 1:
@prefix foaf: < .
@prefix : < .
:people1 foaf:name "Tim BL" .
:people2 foaf:name "James" .
DATASET 2:
< < < .
QUERY:
PREFIX foaf: <
SELECT ?name FROM <
WHERE {
< foaf:knows ?person .
SERVICE <
{ ?person foaf:name ?name . } }
This query, on the datasets provided above, has one solution:
RESULT:
Name
James

20. SPARQL: Federated Query
An example query with OPTIONAL to two remote SPARQL endpoints
Consider we want to query people and optionally obtain their interests and the names of people they know
Data in the default graph at remote SPARQL endpoint: At
DATASET 1:
@prefix foaf: < .
@prefix : < .
:people1 foaf:name "Tim BL" .
:people2 foaf:name "James" .
:people3 foaf:name "Jerome" .
:people3 foaf:interest < . At
DATASET 2:
@prefix foaf: < .
@prefix : < .
:people1 foaf:knows :people21 .
:people21 foaf:name " Chris" .
:people3 foaf:knows :people22 .
:people22 foaf:name “Frank" .
QUERY:
PREFIX foaf: <
SELECT ?person ?interest ?known
WHERE {
SERVICE <
?person foaf:name ?name .
OPTIONAL { ?person foaf:interest ?interest .
SERVICE <
{ ?person foaf:knows ?known . } }
This query, on the datasets provided above, has three solutions:
RESULT:
person interest known
Tim BL
James
Jerome <http…rdb2rdf/> <http…people22>

21. SPARQL Update
Graph Store: an editable repository of RDF graphs managed by a single service
Update service: a service (often referred to by the informal term SPARQL endpoint) that accepts and processes update requests
Similarly to RDF dataset a Graph Store contains one (unnamed) slot holding a default graph and zero or more named slots holding named graphs
Formal definition: Graph Store
The Graph Store can be viewed as a mutable RDF Dataset.
GS = {DG, (iri1, G1), ... , (irin, Gn) }
Where the default graph DG is the RDF graph associated with the unnamed slot
n ≥ 0 and for each 1 ≤ i ≤ n, Gi is an RDF graph associated with the named slot identified by IRI irii
all IRIs are distinct, i.e., i≠j implies irii≠irij

22. SPARQL Update
Update operations can specify graph(s) to be edited. Without such specification an operation is performed on the default graph
The unnamed or default graph may refer to a separate graph, a graph describing the named graphs, a representation of a union of other graphs, and so on
Unlike an RDF Dataset, named graphs can be added to or deleted from a Graph Store
A Graph Store can keep local copies of RDF graphs defined elsewhere on the Web and modify those copies independently of the original graph
SPARQL Update supports two categories of update operations on a Graph Store
Graph Update - addition and removal of triples from some graphs
Graph Management - creating and deletion of graphs and the graph update operations (to add, move, and copy graphs) for managing graphs
In the case where there is one unnamed graph and no named graphs, SPARQL Update can be used
as a graph update language (as opposed to a Graph Store update language)

23. References SPARQL Spec. (2008). W3C Recommendation.
SPARQL 1.1 Federated Query (2013). W3C Recommendation.
SPARQL 1.1 Update (2013). W3C Recommendation.
Turtle Terse RDF Triple Language (2013). 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.