XML as a Query Language

XML Query is a series of specifications, still at Working Draft stage, that primarily include the XML Query Requirements, XML Query Data Model, and XML Query Algebra. Each of these specifications identifies a specific aspect of the XML Query language. In addition to these three working drafts ( released February, 2001) is the XML Query Use Cases working draft, which provides extensive examples of how and where XML Queries can be used.

The XML Query Working Group has three goals which are expressed in the XML Query Requirements working draft: to produce a data model for XML documents, to produce a set of query operators on that data model, and to produce a query language based on these query operators. The data model being developed for XML Queries is based on the W3C XML Infoset. XML Queries work with a single document or with a fixed collection of documents. The primary function of a Query is to select either a whole or subtree of a document based upon the content and structure of that document. Once a Query finds the information it is looking for, it then writes that information to a new document.

In order to make this search-and-rewrite operation work best, XML Queries have a variety of requirements that the language must meet in order to fulfill all of its expectations. The following are some of the requirements:

• XML Queries must be readable and writable by humans.
• XML Queries must be written in valid XML syntax.
• XML Queries must be defined independently of any protocols that it might be used with. In other words, XML Queries can't be dependent upon SQL or another query language to provide their functionality.
• XML Queries must return errors that occur while a query is being processed.
• The XML Query language must be expandable so that future versions and implementations can add further capability to Queries.
• XML Queries support a finite number of data instances. In other words, you can have your Query return the first five records that match your query or all of them.
• XML Queries must support namespaces.
• XML Queries must support XML 1.0 data, as well as XML Schema complex and simple types.
• XML Queries must support references between XML documents, as well as references within a single XML document.
• XML Queries must work correctly, whether or not an XML Schema or DTD is available.

Questionnaire:

• What are XML queries?
• What are the requirements of XML Queries?