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XQuery from the Experts: A Guide to the W3C XML Query Language by Philip Wadler, Jim Tivy, Jérôme Siméon, Michael Rys, Jonathan Robie, Michael Kay, Mary Fernández, Denise Draper, Don Chamberlin, Howard Katz - Editor

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Variable Definitions

A query can define a variable in the prolog. Such a variable is available at any point after it is declared. For instance, if access to the titles of books is used several times in a query, it can be provided in a variable definition:

define variable $titles { doc("books.xml")//title }

To avoid circular references, a variable definition may not call functions that are defined prior to the variable definition.

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