As in SQL, the beginning delimiter of a comment is a double hyphen and the ending delimiter is a newline character.
The input and output of a query are instances of a data model called the XML Query Data Model [Query Data Model].
The Query Working Group has identified a requirement for both a human-readable query syntax and an XML-based query syntax.
XQuery is designed to meet the first of these requirements.
XML is an extremely versatile markup language, capable of labeling the information content of diverse data sources including structured and semi-structured documents, relational databases, and object repositories.
A query language that uses the structure of XML intelligently can express queries across all these kinds of data, whether physically stored in XML or viewed as XML via middleware.