XSLT - What is <xsl:key>?

What is <xsl:key>?

  • <xsl:key> is used to define an index (like a lookup table) in XSLT.

  • Instead of searching the entire XML repeatedly with XPath, you can define a key and then access nodes quickly with the key() function.

  • It’s mainly used for grouping and fast lookups.

Syntax

<xsl:key name="keyName" match="pattern" use="expression"/>

  • name → the name of the key (used when calling key()).

  • match → XPath pattern that selects the nodes to index.

  • use → expression that defines the value(s) of the key (what you’ll look up with).

Example XML

<library>
    <book>
        <title>XML Basics</title>
        <author>Alice</author>
    </book>
    <book>
        <title>XSLT in Action</title>
        <author>Bob</author>
    </book>
    <book>
        <title>Advanced XML</title>
        <author>Alice</author>
    </book>
</library>

Defining a Key

We want to index books by author.

<xsl:key name="booksByAuthor" match="book" use="author"/>

  • This says:
    “For each <book>, create an index entry where the key is the author’s name and the value is the <book> node.”

Using the Key

<xsl:template match="/">
    <html>
        <body>
            <h2>Books by Alice</h2>
            <ul>
                <!-- Lookup all books where author = 'Alice' -->
                <xsl:for-each select="key('booksByAuthor', 'Alice')">
                    <li><xsl:value-of select="title"/></li>
                </xsl:for-each>
            </ul>
        </body>
    </html>
</xsl:template>

Output

<html>
  <body>
    <h2>Books by Alice</h2>
    <ul>
      <li>XML Basics</li>
      <li>Advanced XML</li>
    </ul>
  </body>
</html>

Why use <xsl:key>?

  1. Performance → instead of scanning the whole document every time, key() gives fast access.

  2. Grouping → often used with generate-id() and key() to group elements by some property (like grouping books by author).

  3. Reusability → once defined, you can use the key multiple times in your XSLT.