<?xml version="1.0" encoding="iso-8859-1"?> <?xml-stylesheet href="../make-menu.xsl" type="text/xsl"?><html> <head> <this-is section="xsl-elements" page="key" subpage=""/> <!-- Generated at 2011-12-09T20:47:22.916Z--><title>Saxonica: XSLT and XQuery Processing: xsl:key</title> <meta name="coverage" content="Worldwide"/> <meta name="copyright" content="Copyright Saxonica Ltd"/> <meta name="title" content="Saxonica: XSLT and XQuery Processing: xsl:key"/> <meta name="robots" content="noindex,nofollow"/> <link rel="stylesheet" href="../saxondocs.css" type="text/css"/> </head> <body class="main"> <h1>xsl:key</h1> <p>The <code>xsl:key</code> element is used at the top level of the stylesheet to declare an attribute, or other value, that may be used as a key to identify nodes using the <code>key()</code> function within an expression. Each xsl:key definition declares a named key, which must match the name of the key used in the <code>key()</code> function.</p> <p>The set of nodes to which the key applies is defined by a pattern in the match attribute: for example, if <code>match="ACT|SCENE"</code> then every <code>ACT</code> element and every <code>SCENE</code> element is indexed by this key.</p> <p>The value of the key, for each of these matched elements, is determined by the <code>use</code> attribute. This is an expression, which is evaluated for each matched element. If the expression returns a node-set, the typed value of each node in this node-set acts as a key value. For example, if <code>use="AUTHOR"</code>, then each AUTHOR child of the matched element supplies one key value. </p> <p>Note that:</p> <ol> <li content="para"> <p>Keys are not unique: the same value may identify many different nodes</p> </li> <li content="para"> <p>Keys are multi-valued: each matched node may have several (zero or more) values of the key, any one of which may be used to locate that node</p> </li> <li content="para"> <p>Keys can only be used to identify nodes within a single XML document: the key() function will return nodes that are in the same document as the current node.</p> </li> </ol> <p>All three attributes, <code>name</code>, <code>match</code>, and <code>use</code>, are mandatory.</p> <p>The optional <code>collation</code> attribute can be used when comparing strings.</p> <p>Saxon does not yet allow the <code>xsl:key</code> element to contain a sequence constructor in place of the <code>use</code> attribute.</p> <table width="100%"> <tr> <td> <p align="right"><a class="nav" href="matching-substring.xml">Next</a></p> </td> </tr> </table> </body> </html>