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An important application of XML is the interchange of electronic data (EDI) between multiple data sources on the Web. As XML data proliferates on the Web, applications will need to integrate and aggregate data from multiple sources and clean and transform data to facilitate exchange. Data extraction, conversion, transformation, and integration are all well-understood database problems, and their solutions rely on a query language. A query language for XML, called XQL, is discussed, which is suitable for performing the above tasks. XQL is a declarative, "relational complete" query language and is simple enough that it can be optimized. XQL can extract data from existing XML documents and construct new XML documents.