Need help on value comparison using XSLT - Compare 2 parent nodes

Question

I have a XML which have two parent nodes (Base, Sub). I need to write a XSLT to get the values for below condition.

Condition: Need to get different elements in both parents.

Input XML:

<?xml version="1.0" encoding="UTF-8"?>
<Data>
  <Base>
    <Student_ID>1234</Student_ID>
    <Student_ID>1267</Student_ID>
    <Student_ID>1890</Student_ID>
    <Student_ID>5678</Student_ID>
    <Student_ID>6743</Student_ID>
    <Student_ID>8743</Student_ID>
  </Base>
  <Sub>
    <Student_ID>5678</Student_ID>
    <Student_ID>6743</Student_ID>
    <Student_ID>3226</Student_ID>
    <Student_ID>8123</Student_ID>
  </Sub>
</Data>

Expected Output:

<?xml version="1.0" encoding="UTF-8"?>
<Data>
    <Student_ID>1234</Student_ID>
    <Student_ID>1267</Student_ID>
    <Student_ID>1890</Student_ID>
    <Student_ID>8743</Student_ID>
    <Student_ID>3226</Student_ID>
    <Student_ID>8123</Student_ID>
</Data>

Answer 1

In XSLT 3 you could also consider xsl:merge, that would allow you to handle duplicates in each branch; drawback is that merging requires sorting input sequences on merge key(s) so the result of

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
  expand-text="yes"
    exclude-result-prefixes="#all"
    version="3.0">

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

  <xsl:template match="Data">
    <xsl:copy>
      <xsl:merge>
        <xsl:merge-source name="base" select="Base/Student_ID" sort-before-merge="yes">
          <xsl:merge-key select="."/>
        </xsl:merge-source>
        <xsl:merge-source name="sub" select="Sub/Student_ID" sort-before-merge="yes">
          <xsl:merge-key select="."/>
        </xsl:merge-source>
        <xsl:merge-action>
          <xsl:if test="count(current-merge-group('base')) = 0 and count(current-merge-group('sub')) = 1 or count(current-merge-group('base')) = 1 and count(current-merge-group('sub')) = 0">
            <xsl:copy-of select="current-merge-group()"/>
          </xsl:if>
        </xsl:merge-action>
      </xsl:merge>
    </xsl:copy>
  </xsl:template>
  
</xsl:stylesheet>

has the right elements but in sorted order:

<Data>
   <Student_ID>1234</Student_ID>
   <Student_ID>1267</Student_ID>
   <Student_ID>1890</Student_ID>
   <Student_ID>3226</Student_ID>
   <Student_ID>8123</Student_ID>
   <Student_ID>8743</Student_ID>
</Data>

Of course we could re-sort into original input order using

 <xsl:template match="Data">
    <xsl:copy>
     <xsl:variable name="unique-ids" as="element(Student_ID)*">
      <xsl:merge>
        <xsl:merge-source name="base" select="Base/Student_ID" sort-before-merge="yes">
          <xsl:merge-key select="."/>
        </xsl:merge-source>
        <xsl:merge-source name="sub" select="Sub/Student_ID" sort-before-merge="yes">
          <xsl:merge-key select="."/>
        </xsl:merge-source>
        <xsl:merge-action>
          <xsl:if test="count(current-merge-group('base')) = 0 and count(current-merge-group('sub')) = 1 or count(current-merge-group('base')) = 1 and count(current-merge-group('sub')) = 0">
            <xsl:sequence select="current-merge-group()"/>
          </xsl:if>
        </xsl:merge-action>
      </xsl:merge>
     </xsl:variable>
     <xsl:copy-of select="$unique-ids/."/>
    </xsl:copy>
  </xsl:template>