The document discusses LDQL, a query language designed for the web of linked data, emphasizing its formal syntax and semantics. It addresses limitations of existing query languages by separating the selection of relevant data regions from the evaluation of patterns. The paper concludes that LDQL is more expressive than major existing proposals and demonstrates a web-safeness property for practical query execution.

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