XSLT split sorted data into different tables - sorting

Question

I'm trying to format my xml data into two HTML tables. I successfully can sort some dummy data xls:sort, but I can't split up the sorted data into different tables.
My xml:
<a>
<b id="N">text1</b>
<b id="N">text2</b>
<b id="N+1">text3</b>
<b id="N">text4</b>
<b id="N+2">text5</b>
<b id="N+3">text6</b>
<b id="N">text7</b>
<b id="N+2">text8</b>
</a>
N is in this case a number, but I don't know which number. It could be 2 and 55, 3 and 4, 44 and 52 and 78 and 98.
Each number I wish to send to their own table, so the result would be:
<table>
<tr><td>text1</td></tr>
<tr><td>text2</td></tr>
<tr><td>text4</td></tr>
<tr><td>text7</td></tr>
</table>
<table>
<tr><td>text3</td></tr>
</table>
<table>
<tr><td>text5</td></tr>
<tr><td>text8</td></tr>
</table>
<table>
<tr><td>text6</td></tr>
</table>
How can I devide the sorted data into different tables depending on their attribute?
Any pointers would be appreciated.

Answer 1

The standard approach to this kind of problem in XSLT 1.0 is called Muenchian grouping. You define a key that groups your target elements in the way you want
<xsl:key name="bsById" match="b" use="#id" />
then use a trick with generate-id to extract just the first node in each group as a proxy for the group as a whole
<xsl:apply-templates select="b[generate-id()
= generate-id(key('bsById', #id)[1])]"
mode="group">
<xsl:sort select="#id" />
</xsl:apply-templates>
So now the following template would fire once per group, and you can use the key function within it to get all the nodes in the group
<xsl:template match="b" mode="group">
<table>
<!-- extract all the nodes that are grouped with this one -->
<xsl:apply-templates select="key('bsById', #id)">
<!-- you could <xsl:sort> here if you want to sort within groups -->
</xsl:apply-templates>
</table>
</xsl:template>
<xsl:template match="b">
<tr><td>...</td></tr>
</xsl:template>
All the above is fine if that example is your entire XML document, but if there's more than one a element within the document each with its own set of b elements that need grouping independently, then the key needs to be more complex. The usual trick here is to use the generate-id of the parent a node as part of the grouping key value for its b children:
<xsl:key name="bsByParentAndId" match="a/b" use="concat(generate-id(..), '|', #id)" />
and for the Muenchian grouping expression
<xsl:template match="a">
<xsl:apply-templates select="b[generate-id()
= generate-id(key('bsByParentAndId', concat(
generate-id(current()), '|', #id))[1])]"
mode="group"/>
</xsl:template>
For the record, if you could use XSLT 2.0 then it becomes significantly easier. No need to define a complex key, you simply use for-each-group
<xsl:template match="a">
<xsl:for-each-group select="b" group-by="#id">
<xsl:sort select="current-grouping-key()" />
<table>
<xsl:apply-templates select="current-group()" />
</table>
</xsl:for-each-group>
</xsl:template>
<xsl:template match="b">
<tr><td>...</td></tr>
</xsl:template>