=Paper=
{{Paper
|id=None
|storemode=property
|title=Terp: Syntax for OWL-friendly SPARQL Queries
|pdfUrl=https://ceur-ws.org/Vol-614/owled2010_submission_14.pdf
|volume=Vol-614
|dblpUrl=https://dblp.org/rec/conf/owled/SirinBS10
}}
==Terp: Syntax for OWL-friendly SPARQL Queries ==
https://ceur-ws.org/Vol-614/owled2010_submission_14.pdf
Terp: Syntax for OWL-friendly SPARQL queries
Evren Sirin1 , Blazej Bulka1 , and Michael Smith1
Clark & Parsia, LLC, Washington, DC, USA
{evren,blazej,msmith}@clarkparsia.com
1 Introduction
Web Ontology Language (OWL) [5] can be seen as an extension of Resource De-
scription Framework (RDF). The primary exchange syntax for OWL is RDF/XML,
and every OWL ontology can be represented as an RDF graph. But there is
no standard query language specifically for OWL ontologies. The most com-
monly used Semantic Web query language is SPARQL [7], which is intended
to be used for RDF. Roughly speaking, SPARQL is specified as queries match-
ing RDF graphs with simple RDF entailment. However, it allows this definition
to be extended to OWL entailment. A semantics for SPARQL compatible with
OWL DL has been defined in SPARQL-DL [8] and a similar formalization of
OWL-compatible SPARQL semantics is being developed by the W3C’s SPARQL
Working Group as part of SPARQL 1.1.1
The semantics extension of SPARQL allows one to query OWL ontologies
and get the expected results with respect to OWL entailments. However, writing
SPARQL queries that involve complex OWL expressions ranges from challenging
to unpleasant because SPARQL query syntax is based on Turtle [1], which isn’t
intended for OWL. SPARQL queries against OWL data have to encode the RDF
serialization of OWL expressions: these queries are typically verbose, difficult to
write, and difficult to understand.
In this paper we present Terp, a new syntax that combines Turtle and Manch-
ester syntaxes to provide maximum legibility and conciseness when querying
OWL with SPARQL. More precisely, Terp syntax allows class, property, and data
range expressions, expressed in Manchester syntax, to be used inside SPARQL
queries. In this paper, we provide examples to demonstrate how Terp reuses
existing features from well-known syntaxes to make SPARQL queries of OWL
data more concise and more legible.
2 Terp Syntax
SPARQL syntax provides many Turtle-derived shortcuts for writing concise
queries. Such features are especially useful for instance queries. The following
query against Wine ontology2 shows several feature of the SPARQL syntax.3
1
http://www.w3.org/TR/sparql11-entailment/
2
http://www.w3.org/TR/2004/REC-owl-guide-20040210/wine.rdf
3
We omit prefix declarations in SPARQL queries due to space constraints.
CEUR
ceur-ws.org
Workshop ISSN 1613-0073
Proceedings
SELECT ?wine ?flavor WHERE {
?wine a :FrenchWine , :RedWine ;
:hasFlavor ?flavor
}
Ex. 1. [SPARQL] Find the flavors of red wines produced in France.
This query uses several Turtle shortcuts: 1. the Turtle keyword a instead of
rdf:type qname, 2. object lists separated by comma when subject and predicate
of triples is the same, and 3. predicate-object lists separated by semi-colon when
only the subject of triples is the same.
But the shortcuts provided by Turtle are inadequate when OWL expressions
are used inside SPARQL queries.4 The following example shows the effect of
using simple OWL constructs such as intersection and cardinality restrictions.
SELECT ?wine WHERE {
?wine a [ owl:intersectionOf (
wine:Wine
[ owl:onProperty wine:madeFromGrape ;
owl:minCardinality 2
])]
}
Ex. 2. [SPARQL] Find wine instances that are made of at least two grapes.
Example 2 is very verbose even though it uses Turtle’s bnode and list short-
cuts. And the OWL keywords are mixed with the domain elements making it
harder to read the query.
Manchester syntax [2] is commonly used for concisely writing OWL expres-
sions. It was originally designed to express class expressions in OWL 1 and then
later extended to write entire OWL 2 ontologies. It is used in RDF/OWL editors
such as Protégé 4 and TopBraid Composer.
Terp extends SPARQL syntax by allowing class, property, and data range
expressions expressed in Manchester syntax to be used in queries. The following
example shows how this combination results in a much more concise query.
SELECT ?wine WHERE {
?wine a ( wine:Wine and wine:madeFromGrape min 2 )
}
Ex. 3. [Terp] Example 2 written in Terp syntax.
Terp is designed to support everything in SPARQL syntax, including all
of the abbreviations of Turtle, to which it adds Manchester syntax features to
represent OWL expressions. The following example shows a valid Terp query
that uses several features of SPARQL and Manchester syntaxes.
The grammar for Terp can be found online.5 This grammar is nearly an
exact merge of SPARQL grammar with Manchester syntax grammar. As a result,
4
This isn’t really a criticism of Turtle’s design, since representing OWL constructs
compactly was not a Turtle design desideratum.
5
http://clark-parsia.svn.cvsdude.com/pellet-devel/tags/release-2.1.0/
query/antlr/SparqlOwl.g
SELECT ?mealCourse ?label WHERE {
?mealCourse rdfs:subClassOf
food:MealCourse ,
food:hasDrink some (wine:Wine and
wine:hasBody value wine:Full)
OPTIONAL {
?mealCourse rdfs:label ?label
}
FILTER ( ?mealCourse != owl:Nothing )
}
ORDER BY ?label
Ex. 4. [Terp] Find meal courses that go with full-bodied wines, optionally find the
associated label. Filter owl:Nothing from results and order by labels.
Manchester syntax expressions can appear in the subject and object position of
SPARQL triple patterns. Ultimately, any Terp query can be translated to pure
SPARQL queries in a straight-forward way by using the translation rules for
Manchester syntax [2].6
There are a few cases where ambiguity arises due to the combination of
grammars. For example, the use of parentheses to indicates lists in SPARQL and
nesting in Manchester syntax. If the ambiguity cannot be resolved by the context,
Terp grammar assumes the SPARQL intention, primarily because SPARQL is
known to more people (or so we reasonably assume) than Manchester.
3 Implementation
Experimental Terp support is available in Pellet version 2.1.7 The Pellet distri-
bution contains many more examples of Terp queries. Terp can be used from
the Pellet command-line to execute queries written in Terp syntax. Terp can
also be accessed programmatically: it reads Terp queries and generates standard
SPARQL queries that can be executed by any OWL-aware SPARQL endpoint.
4 Related Work
There have been several efforts to create a query language for OWL. SPAR-
QLAS8 is one example where OWL functional syntax with variables and Manch-
ester syntax like abbreviations is used to encode queries. Similar to Terp, there
is a well-defined translation from SPARQLAS queries to pure SPARQL. SPAR-
QLAS certainly makes writing OWL expressions easier than in standard SPARQL;
however, it supports only conjunctive queries without any SPARQL operators
(i.e., UNION, OPTIONAL, or FILTER).
OWLLink [4] protocol provides a query language that is intended to be used
for OWL. The OWLLink core provides a set of general requests, called basic
6
Manchester syntax defines a mapping to OWL 2 functional syntax for which an RDF
mapping is defined.
7
http://clarkparsia.com/pellet
8
http://code.google.com/p/twouse/wiki/SPARQLAS
asks, for retrieving entailments and a query extension for more complex union of
conjunctive queries. There is both a functional syntax and XML syntax binding
for these queries. However, OWLLink is not designed to improve the concision
or legibility of SPARQL queries for OWL.
SQWRL (Semantic Query-enhanced Web Rule Language) [6] is another query
language for OWL; it’s based on human-readable syntax of SWRL [3]. In princi-
ple, the SWRL specification allows class expressions in human-readable syntax,
but there is no formal grammar about how this can be done. SWRL syntax is
only suitable for instance queries and schema queries need to be handled by
SQWRL built-in function extensions.
5 Conclusions and Future Work
In this paper, we provide a brief description of Terp syntax which allows us-
ing Manchester syntax expressions in SPARQL for concise and readable OWL
queries. Terp is mainly intended to be easy to read and write for humans. Since
Terp queries can be translated to SPARQL queries, this syntax can be used in
conjunction with standard SPARQL Protocol to communicate with SPARQL
endpoints, since the translation can be done on either client or server side. Terp
can be useful even if SPARQL queries are auto-generated through a UI, espe-
cially when those queries need to be inspected or debugged by people.
Terp syntax is still evolving and there are several OWL 2 features such as
negative property assertions and axiom annotations that it does not provide
syntactic shortcuts. Furthermore, there are several extensions possible for Terp
such as allowing Manchester syntax keywords in predicate position (e.g. use
equivalentTo to replace owl:equivalentClass and owl:equivalentProperty)
or additional syntactic sugar for some frequently used OWL 2 constructs.
References
1. D. Beckett and T. Berners-Lee. Turtle - Terse RDF Triple Language. W3C Team
Submission, 2008.
2. M. Horridge and P. F. Patel-Schneider. OWL 2 web ontology language manchester
syntax. W3C Working Group Note, 2009.
3. I. Horrocks, P. F. Patel-Schneider, H. Boley, S. Tabet, B. Grosof, and M. Dean.
SWRL: A semantic web rule language combining OWL and RuleML. W3C, 2004.
4. T. Liebig, M. Luther, and O. Noppens. The OWLlink Protocol. In Proc. of 6th
OWL: Experiences and Directions Workshop (OWLED2009), 2009.
5. B. Motik, P. F. Patel-Schneider, and B. Parsia. OWL 2 web ontology language
structural specification and functional-style syntax. W3C Recommendation, 2009.
6. M. J. O’Connor and A. K. Das. SQWRL: A Query Language for OWL. In Proc. of
6th OWL: Experiences and Directions Workshop (OWLED2009), 2009.
7. E. Prud’hommeaux and A. Seaborne. SPARQL Query Language for RDF. W3C
Recommendation, 2008.
8. E. Sirin and B. Parsia. SPARQL-DL: SPARQL Query for OWL-DL. In Proc. of
3rd OWL: Experiences and Directions Workshop (OWLED2007), 2007.