XSLT Grouping and subgrouping

Question

I have the following code:

    <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="html" />

  <xsl:key name="categories" match="Category" use="." />
  <xsl:key name="clients" match="Category" use ="Category/Client" />

  <xsl:template match="/">
    <ul id="red" class="treeview-red">
      <xsl:for-each select="/Promotions/Promotion/Category[  
        generate-id(.) = generate-id(key('categories', .)[1])  
      ]">
        <xsl:variable name="cname" select="." />

        <li>
          <span>
            <xsl:value-of select="$cname" />
          </span>

          <xsl:for-each select="/Promotions/Promotion[Category=$cname]">
            <ul>
              <li>
                <span>
                  <xsl:value-of select="Client" />
                </span>
              </li>
              <ul>
                <li>
                  <span>
                    <xsl:value-of select="Title" />
                  </span>
                </li>
              </ul>
            </ul>
          </xsl:for-each>
        </li>
      </xsl:for-each>

    </ul>
  </xsl:template>
</xsl:stylesheet>

My XML:

<Promotions>
  <Promotion>
    <Category>Arts &amp; Entertainment</Category>
    <Client>Client 1</Client>
    <Title>Get your Free 2</Title>
  </Promotion>
  <Promotion>
    <Category>Arts &amp; Entertainment</Category>
    <Client>Client 1</Client>
    <Title>Get your Free 4</Title>
  </Promotion>
  <Promotion>
    <Category>Arts &amp; Entertainment</Category>
    <Client>Client 1</Client>
    <Title>Get your Free 5</Title>
  </Promotion>
  <Promotion>
    <Category>Community &amp; Neighborhood</Category>
    <Client>Client 2</Client>
    <Title>Get your Free 1</Title>
  </Promotion>
  <Promotion>
    <Category>Education</Category>
    <Client>Client 3</Client>
    <Title>Get Your Free 3</Title>
  </Promotion>
</Promotions>

It outputs the following:

<ul id="red" class="treeview-red">
  <li><span>Arts &amp; Entertainment</span><ul>
      <li><span>Client 1</span></li>
      <ul>
        <li><span>Get your Free 2</span></li>
      </ul>
    </ul>
    <ul>
      <li><span>Client 1</span></li>
      <ul>
        <li><span>Get your Free 4</span></li>
      </ul>
    </ul>
    <ul>
      <li><span>Client 1</span></li>
      <ul>
        <li><span>Get your Free 5</span></li>
      </ul>
    </ul>
  </li>
  <li><span>Community &amp; Neighborhood</span><ul>
      <li><span>Client 2</span></li>
      <ul>
        <li><span>Get your Free 1</span></li>
      </ul>
    </ul>
  </li>
  <li><span>Education</span><ul>
      <li><span>Client 3</span></li>
      <ul>
        <li><span>Get Your Free 3</span></li>
      </ul>
    </ul>
  </li>
</ul>

I would like the output to be grouped by category first then by client for each category, any insight to this would be great:

<ul id="red" class="treeview-red">
  <li><span>Arts &amp; Entertainment</span><ul>
      <li><span>Client 1</span></li>
      <ul>
        <li><span>Get your Free 2</span></li>
      </ul>
      <ul>
        <li><span>Get your Free 4</span></li>
      </ul>
      <ul>
        <li><span>Get your Free 5</span></li>
      </ul>
    </ul>
  </li>
  <li><span>Community &amp; Neighborhood</span><ul>
      <li><span>Client 2</span></li>
      <ul>
        <li><span>Get your Free 1</span></li>
      </ul>
    </ul>
  </li>
  <li><span>Education</span><ul>
      <li><span>client 3</span></li>
      <ul>
        <li><span>Get Your Free 3</span></li>
      </ul>
    </ul>
  </li>
</ul>

Basically after grouped by category, I only want to see one client under the category with each promotion for that client in that category.

Answer 1

Without seeing the XML input it is hard to suggest changes to your stylesheet so currently all I can do is to look at http://www.biglist.com/lists/xsl-list/archives/200101/msg00070.html as an example of multi level grouping with XSLT 1.0.

[edit]: Here is how you could apply two level Muenchian grouping:

<xsl:stylesheet
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  version="1.0">

  <xsl:output method="html" indent="yes"/>

  <xsl:key name="k1" match="Promotion" use="Category"/>
  <xsl:key name="k2" match="Promotion" use="concat(Category, '|', Client)"/>

  <xsl:template match="Promotions">
    <ul id="red" class="treeview-red">
      <xsl:for-each select="Promotion[generate-id() = generate-id(key('k1', Category)[1])]">
        <li>
          <span>
            <xsl:value-of select="Category"/>
          </span>
          <xsl:for-each select="key('k1', Category)[generate-id() = generate-id(key('k2', concat(Category, '|', Client))[1])]">
            <ul>
              <li>
                <span>
                  <xsl:value-of select="Client"/>
                </span>
                <xsl:for-each select="key('k2', concat(Category, '|', Client))">
                  <ul>
                    <li>
                      <span>
                        <xsl:value-of select="Title"/>
                      </span>
                    </li>
                  </ul>
                </xsl:for-each>
              </li>
            </ul>
          </xsl:for-each>
        </li>
      </xsl:for-each>
    </ul>
  </xsl:template>

</xsl:stylesheet>