I used this slideset to present our research paper at the 14th Int. Semantic Web Conference (ISWC 2015). Find a preprint of the paper here: http://olafhartig.de/files/HartigPerez_ISWC2015_Preprint.pdf

1. LDQL: A Query Language
for the Web of Linked Data
Olaf Hartig¹ and Jorge Pérez²
¹Hasso Plattner Institute, University of Potsdam, Germany
²Department of Computer Science, Universidad de Chile

2. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 2
Linked Data Query Processing

3. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 3
Research Question
What language
can we use to express queries
(in a declarative manner)
?

4. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 4
Existing Proposals 1/2
● Basic Graph Patterns
– Bounded method [Bouquet, Ghidini, Serafini 2009]
– Navigational method [Bouquet, Ghidini, Serafini 2009]
– Direct access method [Bouquet, Ghidini, Serafini 2009]
– Matching-based query semantics [Hartig 2011]
– Family of authoritativeness-based query semantics
[Harth and Speiser 2012]
– Different inference-based query semantics
[Umbrich et al. 2012]

5. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 5
Existing Proposals 2/2
● SPARQL 1.0
– Full-Web query semantics [Hartig 2012]
– Family of reachability-based semantics [Hartig 2012]
● cALL-semantics, cNONE-semantics, cMATCH-semantics, …
● SPARQL 1.1 Property Paths Patterns
– Context-based query semantics [Hartig and Pirrò 2015]
● NautiLOD [Fionda, Gutierrez, and Pirrò 2012]
● LDPath (no formal semantics) [Schaffert et al. 2012]

6. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 6
Limitation of Existing Proposals
● No separation of the following two tasks:
– Select query-relevant regions of the Web of Linked Data
– Evaluate pattern over the data in the selected regions
● For instance, impossible to express queries such as:
– Take all Linked Data reached by following
two foaf:knows links starting from URI u,
and evaluate a given SPARQL pattern
over the union of this data.

7. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 7
General Idea of LDQL
Query
Specification of
query-relevant
region
( N , Q )

8. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 8
Our Contributions
● Formal syntax and semantics of LDQL
– Including some rewrite rules (i.e., equivalences)
● Study Web safeness of LDQL queries
– Show a decidable syntactic property of LDQL queries
for which a complete execution is feasible in practice
● Compare LDQL with existing proposals
– Show that LDQL is strictly more expressive
? ?
?
? ?
??
?

9. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 9
Existing Proposals 2/2
● SPARQL 1.0
– Full-Web query semantics [Hartig 2012]
– Family of reachability-based semantics [Hartig 2012]
● cALL-semantics, cNONE-semantics, cMATCH-semantics, …
● SPARQL 1.1 Property Paths Patterns
– Context-based query semantics [Hartig and Pirrò 2015]
● NautiLOD [Fionda, Gutierrez, and Pirrò 2012]
● LDPath (no formal semantics) [Schaffert et al. 2012]
√
√
√ √ √

10. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 10
Our Contributions
● Formal syntax and semantics of LDQL
– Including some rewrite rules (i.e., equivalences)
● Study Web safeness of LDQL queries
– Show a decidable syntactic property of LDQL queries
for which a complete execution is feasible in practice
● Compare LDQL with existing proposals
– Show that LDQL is strictly more expressive
? ?
?
? ?
??
?
Let's have a brief look!

11. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 11
General Idea of LDQL
Query
( N , Q )
Specification of
query-relevant
region

12. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 12
General Idea of LDQL
SPARQL 1.1
graph pattern
Link Path Expression
(Nested regular
expressions over
an alphabet of so
called link patterns)
Query
( N , Q )
Specification of
query-relevant
region

13. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 13
Link Patterns
● Each βi is either:
– a URI, or
– a wildcard denoted by _, or
– a placeholder for the context URI denoted by +
● β3 may also be a literal
● Example:
( β1, β2, β3 )
( +, foaf:knows, _ )

14. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 14
Link Graph
( ua, foaf:knows, ub )
( uc, foaf:knows, ua )
doc(ua)
…
doc(ub)
...
doc(uc)
( +, foaf:knows, _ )

15. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 15
Link Graph
( ua, foaf:knows, ub )
( uc, foaf:knows, ua )
doc(ua)
…
doc(ub)
...
doc(uc)
( +, foaf:knows, _ )
( ua, foaf:knows, ub ), ub ( uc, foaf:knows, ua ), uc( ua, foaf:knows, ub )
( uc, foaf:knows, ua )

16. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 16
Link Pattern Matching
( ua, foaf:knows, ub )
( uc, foaf:knows, ua )
doc(ua)
…
doc(ub)
...
doc(uc)
( +, foaf:knows, _ )
( ua, foaf:knows, ub ), ub ( uc, foaf:knows, ua ), uc
A link graph edge labeled with RDF triple (x1, x2, x3) and URI u
matches a link pattern (β1, β2, β3) in the context of URI uctx if:
•  i ∈ {1,2,3} such that βi = _ and xi = u, and
•  i ∈ {1,2,3} (i) βi = xi , (ii) βi = _ , or (iii) βi = + and xi = uctx

17. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 17
Link Pattern Matching
( ua, foaf:knows, ub )
( uc, foaf:knows, ua )
doc(ua)
…
doc(ub)
...
doc(uc)
( +, foaf:knows, _ )
( ua, foaf:knows, ub ), ub ( uc, foaf:knows, ua ), uc
A link graph edge labeled with RDF triple (x1, x2, x3) and URI u
matches a link pattern (β1, β2, β3) in the context of URI uctx if:
•  i ∈ {1,2,3} such that βi = _ and xi = u, and
•  i ∈ {1,2,3} (i) βi = xi , (ii) βi = _ , or (iii) βi = + and xi = uctx
√ X

18. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 18
(Algebraic) Syntax of LDQL
q := (lpe, P) | (lpe an LPE, P a SPARQL pattern)
(q AND q) |
(q UNION q) |
πV q | (V a set of variables)
(SEED U q) | (U a set of URIs)
(SEED ?v q) (?v a variable)
lpe := ε | lp | lpe/lpe | lpe|lpe | lpe* | [lpe] | (?v, q)

19. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 19
(Algebraic) Syntax of LDQL
q := (lpe, P) | (lpe an LPE, P a SPARQL pattern)
(q AND q) |
(q UNION q) |
πV q | (V a set of variables)
(SEED U q) | (U a set of URIs)
(SEED ?v q) (?v a variable)
lpe := ε | lp | lpe/lpe | lpe|lpe | lpe* | [lpe] | (?v, q)
Example: Take all Linked Data reached by following two
foaf:knows links starting from URI u, and evaluate
SPARQL pattern P over the union of this data.
( (+,foaf:knows, _) /(+,foaf:knows, _) , P )
with seeds S = {u }

20. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 20
(Algebraic) Syntax of LDQL
q := (lpe, P) | (lpe an LPE, P a SPARQL pattern)
(q AND q) |
(q UNION q) |
πV q | (V a set of variables)
(SEED U q) | (U a set of URIs)
(SEED ?v q) (?v a variable)
lpe := ε | lp | lpe/lpe | lpe|lpe | lpe* | [lpe] | (?v, q)
Example: Take all Linked Data reached by following two
foaf:knows links starting from URI u, and evaluate
SPARQL pattern P over the union of this data.
( (+,foaf:knows, _) /(+,foaf:knows, _) , P )
with seeds S = {u }

21. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 21
(Algebraic) Syntax of LDQL
q := (lpe, P) | (lpe an LPE, P a SPARQL pattern)
(q AND q) |
(q UNION q) |
πV q | (V a set of variables)
(SEED U q) | (U a set of URIs)
(SEED ?v q) (?v a variable)
lpe := ε | lp | lpe/lpe | lpe|lpe | lpe* | [lpe] | (?v, q)
Example: Take all Linked Data reached by following two
foaf:knows links starting from URI u, and evaluate
SPARQL pattern P over the union of this data.
( (+,foaf:knows, _) /(+,foaf:knows, _) , P )
with seeds S = {u }

22. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 22
(Algebraic) Syntax of LDQL
q := (lpe, P) | (lpe an LPE, P a SPARQL pattern)
(q AND q) |
(q UNION q) |
πV q | (V a set of variables)
(SEED U q) | (U a set of URIs)
(SEED ?v q) (?v a variable)
lpe := ε | lp | lpe/lpe | lpe|lpe | lpe* | [lpe] | (?v, q)
For every LDQL query q',
there exists a semantically equivalent LDQL query q''
such that
every LPE in q'' consists only of ε and lpe* and (?v, q).

23. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 23
(Algebraic) Syntax of LDQL
q := (lpe, P) | (lpe an LPE, P a SPARQL pattern)
(q AND q) |
(q UNION q) |
πV q | (V a set of variables)
(SEED U q) | (U a set of URIs)
(SEED ?v q) (?v a variable)
Example: ( (lpe1 , P1) AND (SEED ?x q2) )
where P1 is: ((?x,p,?y) FILTER (?y > 3))
lpe := ε | lp | lpe/lpe | lpe|lpe | lpe* | [lpe] | (?v, q)

24. LDQL: A Query Language for the Web of Linked Data – Olaf Hartig and Jorge Peréz – ISWC 2015 24
Summary
● Query language for the Web of Linked Data
● Main feature: separation of ...
… navigational components to select
regions of the Web of Linked Data, and
… query patterns to be matched in the
data of the selected regions
● In the paper we study LDQL
– Web-safeness property
– LDQL is strictly more expressive
than the major existing proposals