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XML (Extensible Markup Language) has emerged as the standard data-exchange format for Internet-based business applications. These applications introduce a new set of data management requirements involving XML. However, for the foreseeable future, a significant amount of business data will continue to be stored in relational database systems. Thus, a bridge is needed to satisfy the requirements of these new XML-based applications while still using relational database technology. This paper describes the design and implementation of the XTABLES middleware system, which we believe achieves this goal. In particular, XTABLES provides a general framework to create XML views of relational data, query XML views, and store and query XML documents using a relational database system. Some of the novel features of the XTABLES architecture are that it 1) provides users with a single XML query language for creating and querying XML views of relational data, 2) executes queries efficiently by pushing most computation down to the relational database engine, c) allows users to query seamlessly over relational data and meta-data, and 4) allows users to write queries that span XML documents and XML views of relational data.
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