xsl:key

Declares a named index for fast lookup with the XPath key() function.

<xsl:key name="emp-by-dept" match="employee" use="@department"/>

For each node in the source document that matches match, the index stores it under the key value(s) computed by use. A subsequent key('emp-by-dept', 'Sales') returns all employees in the Sales department in O(1) plus result size, instead of O(n) per lookup.

Multiple xsl:key declarations with the same name are merged into a single index. With composite="yes", the use expression must produce a sequence and the entire sequence is treated as a composite key.

Attributes

Example

Looking up employees by department:

<xsl:key name="emp-by-dept" match="employee" use="@department"/>

<xsl:template match="/">
  <xsl:for-each select="distinct-values(//employee/@department)">
    <dept name="{.}">
      <xsl:for-each select="key('emp-by-dept', .)">
        <emp><xsl:value-of select="@name"/></emp>
      </xsl:for-each>
    </dept>
  </xsl:for-each>
</xsl:template>