[Semantic Web]The expressivity of RDF and RDF Schema

读书笔记,  电脑与网络 Lee 发表于 2006/2/14 18:46:31 　 The expressivity of RDF and RDF Schema is deliberately very limited: RDF is (roughly) limited to binary ground predicates, and RDF Schema is (roughly) limited to a subclass hierarchy and a property hierarchy, with domain and range definitions of these properties. However, the Web Ontology Working Group of W3C identified a number of characteristic use-cases for the Semantic Web that would require much more expressiveness than RDF and RDF Schema offer. RDF and RDFS allow the representation of some ontological knowledge. The main modeling primitives of RDF/RDFS concern the organization of vocabularies in typed hierarchies: subclass and subproperty relationships, domain and range restrictions, and instances of classes. However, a number of other features are missing. Here we list a few: • Local scope of properties. rdfs:range defines the range of a property, say eats, for all classes. Thus in RDF Schema we cannot declare range restrictions that apply to some classes only. For example, we cannot say that cows eat only plants, while other animals may eat meat, too. • Disjointness of classes. Sometimes we wish to say that classes are disjoint. For example, male and female are disjoint. But in RDF Schema we can only state subclass relationships, e.g., female is a subclass of person. • Boolean combinations of classes. Sometimes we wish to build new classes by combining other classes using union, intersection, and complement. For example, we may wish to define the class person to be the disjoint union of the classes male and female. RDF Schema does not allow such definitions. • Cardinality restrictions. Sometimes we wish to place restrictions on how many distinct values a property may or must take. For example, we would like to say that a person has exactly two parents, or that a course is taught by at least one lecturer. Again, such restrictions are impossible to express in RDF Schema. • Special characteristics of properties. Sometimes it is useful to say that a property is transitive (like “greater than”), unique (like “is mother of”), or the inverse of another property (like “eats” and “is eaten by”).Thus we need an ontology language that is richer than RDF Schema, a language that offers these features and more. In designing such a language one should be aware of the trade-off between expressive power and efficient reasoning support. Generally speaking, the richer the language is, the more inefficient the reasoning support becomes, often crossing the border of noncomputability. Thus we need a compromise, a language that can be supported by reasonably efficient reasoners while being sufficiently expressive to express large classes of ontologies and knowledge.

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