Ontology Query Languages for Ontology-Based Databases - Data Warehousing Design and Advanced Engineering Applications - page 238

Requirement 3 (Linguistic
Exploitation)

such as OWL restrictions. Example. Create the
class PostDupont defined as all messages of the
user Dupont.

RDF-Schema allows to associate names defined
in different natural languages to classes and prop-
erties. It permits also to define string values in
different natural languages. RQL does not exploit
these features.

CREATE NAMESPACE myview=&http://
www.lisi.ensma.fr/ex-view.rdf#
VIEW rdfs:Class(“PostDupont”)
Property(P, PostDupont,
range(P))
FROM Property{P}
WHERE domain(P) >= sioc:Post
VIEW PostDupont(P), title(P, T),
has_creator(P, C)
FROM sioc:Post{P}.sioc:title{T},
{P}sioc:has_creator{C}
WHERE C = “Dupont”

Requirement 4 (Ontology
Model Extensibility)

The ontology model supported by RQL is com-
posed of the constructors Class and Property. This
ontology model can be extended by specialization
of these two constructors. But, a constructor can
not be added if it does not inherit from Class or
Property. For example, this limitation prevents to
add the Document constructor of PLIB (to describe
a concept by a document) or the Ontology con-
structor of OWL (to regroup all concepts defined
in an ontology). Moreover, if these capabilities are
defined on the data model of RQL, the language
does not provide any operator to use them. Thus,
we can not use RQL to define the OWL AllVal-
uesFrom constructor.

Explanation. In RVL, views (non canonical
concepts) are separated from classes (canonical
concepts). Thus, a new namespace is created for
views (http://www.lisi.ensma.fr/ex-view.rdf).
The first VIEW clause is used to create the view
PostDupont with all properties defined on the class
Post. This is done using a RQL query that searches
all properties defined on Post or on a superclass of
Post (domain(P) >= sioc:Post). The second VIEW
clause is used to compute instances of the view
PostDupont by searching all messages (P) with
their title (T) that have been created by Dupont
(C). Notice that, for conciseness, we have only
retrieved values of the title property. To define
the complete view, other properties values must
be searched.
Thus, RQL fulfil requirement 2. However,
notice that the distinction between canonical and
non canonical concepts forbids the definition of
subsumption relationships between these two kind
of concepts. Thus, it is necessary to reproduces
manually this behaviour (by importing properties
and their values in the NCCO).

Requirements 5 and 6
(Compatibility with the Traditional
Database Architecture)

The syntax of RQL is close to the one of object-
oriented languages. However, it keeps no compat-
ibility with SQL (requirement 5). In addition, RQL
considers all instances as a URI independently
of the classes it belongs to and of its properties
values. Thus, it can not be used to manipulate the
data schema (requirement 6).

Requirement 7 (Ontology & Data
Definition and Manipulation)

As we have seen in requirement 4, RQL does
not provide a syntax to define the ontology