The popularity of the World-Wide Web (WWW) has made it a prime vehicle for disseminating information. The relevance of database concepts to the problems of managing and querying this information has led to a significant body of recent research addressing these problems. Even though the underlying challenge is the one that has been traditionally addressed by the database community how to manage large volumes of data the novel context of the WWW forces us to significantly extend previous techniques. The primary goal of this survey is to classify the different tasks to which database concepts have been applied, and to emphasize the technical innovations that were required to do so.

Database techniques for the World-Wide Web: a survey

Real-scale Semantic Web applications, such as Knowledge Portals and E-Marketplaces, require the management of large volumes of metadata, i.e., information describing the available Web content and services. Better knowledge about their meaning, usage, accessibility or quality will considerably facilitate an automated processing of Web resources. The Resource Description Framework (RDF) enables the creation and exchange of metadata as normal Web data. Although voluminous RDF descriptions are already appearing, sufficiently expressive declarative languages for querying both RDF descriptions and schemas are still missing. In this paper, we propose a new RDF query language called RQL. It is a typed functional language (a la OQL) and relies on a formal model for directed labeled graphs permitting the interpretation of superimposed resource descriptions by means of one or more RDF schemas. RQL adapts the functionality of semistructured/XML query languages to the peculiarities of RDF but, foremost, it enables to uniformly query both resource descriptions and schemas. We illustrate the RQL syntax, semantics and typing system by means of a set of example queries and report on the performance of our persistent RDF Store employed by the RQL interpreter.

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RQL: a declarative query language for RDF

Schema Mapping as Query Discovery

A broad spectrum of data is available on the Web in distinct heterogeneous sources, and stored under different formats. As the number of systems that utilize this heterogeneous data grows, the importance of data translation and conversion mechanisms increases greatly. In this paper we present a new translation system, based on schema-matching, aimed at simplifying the intricate task of data conversion. We observe that in many cases the schema of the data in the source system is very similar to that of the target system. In such cases, much of the translation work can be done automatically, based on the schemas similarity. This saves a lot of effort for the user, limiting the amount of programming needed. We define common schema and data models, in which schemas and data (resp.) from many common models can be represented. Using a rule-based method, the source schema is compared with the target one, and each component in the source schema is matched with a corresponding component in the target schema. Then, based on the matching achieved, data instances of the source schema can be translated to instances of the target schema. We show that our schema-based translation system allows a convenient specification and customization of data conversions, and can be easily combined with the traditional data-based translation languages.

/pdf/using-schema-matching-to-simplify-heterogeneous-data-3jekzgdhfz.pdf

Using Schema Matching to Simplify Heterogeneous Data Translation

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Answering XML Queries on Heterogeneous Data Sources

Due to the development of the World Wide Web, the integration of heterogeneous data sources has become a major concern of the database community. Appropriate architectures and query languages have been proposed. Yet, the problem of data conversion which is essential for the development of mediators/wrappers architectures has remained largely unexplored.In this paper, we present the YAT system for data conversion. This system provides tools for the specification and the implementation of data conversions among heterogeneous data sources. It relies on a middleware model, a declarative language, a customization mechanism and a graphical interface.The model is based on named trees with ordered and labeled nodes. Like semistructured data models, it is simple enough to facilitate the representation of any data. Its main originality is that it allows to reason at various levels of representation. The YAT conversion language (called YATL) is declarative, rule-based and features enhanced pattern matching facilities and powerful restructuring primitives. It allows to preserve or reconstruct the order of collections. The customization mechanism relies on program instantiations: an existing program may be instantiated into a more specific one, and then easily modified. We also present the architecture, implementation and practical use of the YAT prototype, currently under evaluation within the OPAL* project.

Your mediators need data conversion

We present a formal data model for views in Object DataBase Systems (ODBS) as a transformation mechanism for databases Our model relaxes the usual coustraiul, where an object belongs to a single class while using a generalization of referent and eatables a deterministic creation of derived objects with complex object identifiers We define an IQL-like language which enables the manipulation of such referents The view-based transformation is achieved in two steps: an extension of the source instance followed by a projection of the extended instance The extension and projection can be carried ont using four object algebraic operators, namely projection, join-specialization, join and generalization, that specify both the virtual sclaetna and its corresponding virtual instance This simple algebra call express most of the view operators proposed in the literature a ad provides a real restructuring of the source schema and instance

Object Views and Database Restructuring

We present a formal data model for views in Object DataBase Systems (ODBS) which is a continuation and a formalization of the work presented in AK89, AB91, ADdS94, dS95a]. The main contribution of this paper is the formalization of the views as a transformation mechanism for databases using a generalization of referent proposed in dS95b]. The view-based transformation is achieved in two steps: an extension of the source base followed by a projection of the extended base. The extension and projection can be carried out using ve object algebraic operators which are described in detail in this paper. Equally important, this paper highlights the challenges involved supporting our view mechanism. We show how this basic model for views point to new perspectives, namely, extension of OQL, transformation of query on the view in a query on the source schema and base, and nally characterization of the class of view-transformations our algebra captures. Nous pr esentons un mod ele de donn ees formel pour les vues des Syst emes de Bases de Donn ees Objet qui est une extension et une formalisation des travaux pr esent es dans AB91, ADdS94, dS95a]. La principale contribution de cet article est la formalisation des vues comme m ecanisme de transformation de bases en utilisant une g en eralisation du r ef erent propos e dans dS95b]. Cette transformation de vue est eeectu ee en deux etapes: une extension de la base source suivie d'une projection de la base etendue. L'extension et la projection sont eeectu ees en utilisant cinq op erateurs de l'alg ebre, op erateurs d ecrits en d etail dans cet article. De plus, cet article met en avant dii erentes questions soulev ees par notre m ecanisme de vues. Nous mon-trons en particulier comment ce mod ele de base oore de nouvelles perspectives, a savoir, extension de OQL,

Claude Delobel

Papers

Your mediators need data conversion

Object Views and Database Restructuring

Object Views Constructed with an Object Algebra.