Simple Mapper Question

Depends on what type of data A,B and C are.

If they are a small set of known data items that seldom changes then you could use condition functoids
e.g. if firstfield = "A"
This would hardcode the values A, B, and C into the map.

If A,B, and C are part of a large set that does change, i.e. you don't want to hardcode them into the map,
then you need to use a custom Xslt with Muenchian grouping
e.g.

<?xml version="1.0" encoding="utf-16"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:msxsl="urn:schemas-microsoft-com:xslt" exclude-result-prefixes="msxsl" version="1.0">
<xsl:output indent="yes" omit-xml-declaration="yes" version="1.0" method="xml" />
<xsl:key name="recordkey" match="/root/record" use="firstfield"/>
<xsl:template match="/">
<xsl:apply-templates select="/root" />
</xsl:template>
<xsl:template match="/root">
 <root>
  <xsl:for-each select="record">
   <xsl:variable name="group" select="key('recordkey', firstfield)"/>
   <xsl:if test="generate-id($group[1]) = generate-id()">
    <Parent>
     <xsl:attribute name="Value">
      <xsl:value-of select="firstfield"/>
     </xsl:attribute>
     <xsl:for-each select="$group">
      <Child>
       <xsl:attribute name="Value">
        <xsl:value-of select="secondfield"/>
       </xsl:attribute>
      </Child>
     </xsl:for-each>
    </Parent>
   </xsl:if>
  </xsl:for-each>
 </root>
</xsl:template>
</xsl:stylesheet>