Extensible Markup Language (XML) is commonly used for data exchange in message queues among different components, even legacy applications of a heterogeneous and/or distributed software systems. Relational database management systems (RDBMSs) are now often therefore provided with XML interfaces.

Viewing data in XML extracted from RDBMSs is the first step. Following research at the World Wide Web Consortium (W3C), with the languages XQuery and XPath supporting expression of queries, some leading RDBMS vendors (Oracle, IBM, and Microsoft) and even some others involved in message queuing (BEA Systems) and software integration (Software AG) have developed tools to implement XML extractions and queries for RDBMSs.

When expressing XML queries for RDBMSs, the main problem is guidance of the mapping between the hierarchical and the relational models. Theoretical methods, experimental systems and commercial systems each implement different languages with different expressive power. The research of Braganholo et al. provides essential contributions in relation to this theme, being capable of handling heterogeneous groups of repeated elements. As a result of their investigation, they slightly change the focus and denotation of XQuery by introducing the function “table,” by which the mapping can be guided. They introduce a subset of XQuery to define XML views over RDBMSs and an algorithm for the mapping of XML view queries to a set of relational ones. The resulting language, UXQuery, has been determined to express those views, which may be updated without side effects in the underlying RDBMSs with correct update descriptions. Ultimately, Braganholo et al. have generalized the efforts of others in a coherent way. The paper summarizes the theory and the experimental system, PATAXÓ.