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Path, Predicates, and XQuery


In this conclusion to a three-part series on path expressions, you'll learn about complex predicates, dynamic paths, and more. This article is excerpted from chapter four of the book XQuery, written by Priscilla Walmsley (O'Reilly, 2007; ISBN: 0596006349). Copyright 2007 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.

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By: O'Reilly Media
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March 06, 2008
TABLE OF CONTENTS:
  1. · Path, Predicates, and XQuery
  2. · More Complex Predicates
  3. · Dynamic Paths
  4. · Accessing a Single Document
  5. · Setting the Context Node Outside the Query
  6. · Working with the Context Node

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Path, Predicates, and XQuery - Accessing a Single Document
(Page 4 of 6 )

The doc function can be used to open one input document based on its URI. It takes as an argument a single URI as a string, and returns the document node of the resource associated with the specified URI.

Implementations interpret the URI passed to the doc function in different ways. Some, like Saxon, will dereference the URI, that is, go out to the URL and retrieve the resource at that location. For example, using Saxon:

  doc("http://datypic.com/order.xml")

will return the document node of the document that can be found at the URL http:// datypic.com/order.xml.

Other implementations, such as those embedded in XML databases, consider the URIs to be just names. The processor might take the name and look it up in an internal catalog to find the document associated with that name. The doc function is covered in detail in Appendix A.

Accessing a Collection

The collection function returns the nodes that make up a collection. A collection may be a sequence of nodes of any kind, identified by a URI. Exactly how the URI is associated with the nodes is defined by the implementation. For example, one implementation might accept a URI that is the name of a directory on a filesystem, and return the document nodes of the XML documents stored in that directory. Another implementation might associate a URI with a particular database. A third might allow you to specify the URI of an XML document that contains URIs for all the XML documents in the collection.

The function takes as an argument a single URI. For example, the function call:

  collection(http://datypic.com/orders)

might return all the document nodes of the XML documents associated with the collection http://datypic.com/orders. It is also possible to use the function without any parameters, as in collection(), to retrieve a default collection as defined by the implementation.

Some XQuery implementations support a function called input, with no arguments. This function appeared in earlier drafts of the XQuery recommendation but is no longer part of the standard. It is equivalent to calling the collection function with no arguments.


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