By Jonathan Robie
As more and more information is either stored in XML, exchanged in XML, or presented as XML through various interfaces, the ability to intelligently query our XML data sources becomes increasingly important. XML documents are structured documents – they blur the distinction between data and documents, allowing documents to be treated as data sources, and traditional data sources to be treated as documents.
XQL is a query language designed specifically for XML. In the same sense that SQL is a query language for relational tables and OQL is a query language for objects stored in an object database, XQL is a query language for XML documents. The basic constructs of XQL correspond directly to the basic structures of XML, and XQL is closely related to XPath, the common locator syntax used by XSL and XPointers. Since queries, transformation patterns, and links are all based on patterns in structures found in possible XML documents, a common model for the pattern language used in these three applications is both possible and desirable, and a common syntax to express the patterns expressed by that model simplifies the task of the user who must master a variety of XML-related technologies. Although XQL originated before XSL Patterns, there were strong similarities between the two languages, and we have adopted XPath syntax for the constructs which differed. Not all constructs found in XPath were needed for queries, and some constructs used in XQL are not found in XPath, but the two languages share a common subset.
The XQL language described in this paper contains several features not found in previously published versions of the language, including joins, links, text containment, and extensible functions. These new features are inspired in large part by discussions stemming from the W3C QL '98 Workshop, and make it possible to combine information from heterogeneous data sources in powerful ways. Great care has been made to maintain the fundamental simplicity of XQL while adding these features.
This paper is intended as input for the upcoming W3C Query Language Activity, and for the further development of XPath.