XSLT: Select Distinct in XSL 1.0

cThe further one dives into XSLT, it may become necessary to extract a list of unique values from an XML document. This is commonly done in SQL through the SELECT DISTINCT statement, unfortunately, there is no direct equivalent in XSLT 1.0.

In order to perform this sort of functionality, one must leverage some of the more advanced aspects of XSLT including the preceding-sibling:: or another such "axis" as it's known in XSL.

To better understand, lets look at an example.  Given the following XSD snippet:

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <xsd:element name="houseCategory" abstract="true"/>
    <xsd:element name="houseCategoryText" type="xsd:string" substitutionGroup="houseCategory"/>
    <xsd:element name="houseCategoryFlag" type="xsd:boolean" substitutionGroup="houseCategory"/>
    <xsd:element name="housePurchaseDateRepresentation" abstract="true"/>
    <xsd:element name="housePurchaseDate" type="xsd:date" substitutionGroup="housePurchaseDateRepresentation"/>
    <xsd:element name="housePurchaseDateTime" type="xsd:dateTime" substitutionGroup="housePurchaseDateRepresentation"/>
</xsd:schema>

The following XSLT will extract a list of unique substitutionGroup attribute values from above and list them in the output: 

<?xml version="1.0" encoding="UTF-8"?>
  <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0" xmlns:xsd="http://www.w3.org/2001/XMLSchema" >
  <xsl:output method="xml" indent="yes"/>
  <xsl:template match="*">
    <xsl:for-each
        select="/xsd:schema/xsd:element/@substitutionGroup[not(. = ../preceding-sibling::xsd:element/@substitutionGroup/.)]">
        
        <xsl:element name="uniqueSubstitutionGroup">
            <xsl:value-of select="."/>
        </xsl:element>
        
    </xsl:for-each>    
  </xsl:template>
</xsl:stylesheet>

The resulting output would appear something like the following:

<?xml version="1.0" encoding="utf-8"?>
<uniqueSubstitutionGroup>houseCategory</uniqueSubstitutionGroup>
<uniqueSubstitutionGroup>housePurchaseDateRepresentation</uniqueSubstitutionGroup>

The information surrounding this question was sourced in part from information provided on Stack Overflow here. 

XSLT: Namespace From Prefix in XSL 1.0

When transforming XML Schema (XSD) files in XSLT 1.0, it quickly becomes necessary to resolve the namespace from the prefix of any given element or attribute.  Sometimes these elements and attributes are not stored as actual elements and attributes, which makes it necessary to resolve them in a different manner. For example, in the following XSD snippet, iso_639-3:LanguageCodeSimpleType is not a node, element nor is it an attribute, rather it is a text value of the type attribute.

<xsd:schema 
  xmlns:xsd="http://www.w3.org/2001/XMLSchema" 
  xmlns:iso_639-3="http://niem.gov/niem/iso_639-3/2.0">
    <xsd:attribute name="languageCode" 
      type="iso_639-3:LanguageCodeSimpleType"/>
</xsd:schema>

One way to resolve what namespace the iso_639-3 prefix is associated with, is to extract the prefix from the text and use the following simple XSLT 1.0 function/template.

<xsl:template name="namespaceFromPrefix">
  <xsl:param name="sPrefix"/>
  <xsl:value-of select="/*/namespace::*[name() = $sPrefix]"/>
</xsl:template>

If one were to pass in the prefix iso_639-3 the return value would be http://niem.gov/niem/iso_639-3/2.0