Xpath Select values from one node that doesn't exist in another - xslt

Question

<my-courses>
<courses-reqd>
<details>
<subject>Math</subject>
</details>
<details>
<subject>Econ</subject>
</details>
<details>
<subject>Geog</subject>
</details>
<details>
<subject>Phys</subject>
</details>
<detail>
<subject>Zool</subject>
</details>
</courses-reqd>
<courses-taken>
<details>
<subject>Math</subject>
</details>
<details>
<subject>Econ</subject>
</details>
<detail>
<subject>Zool</subject>
</details>
</courses-taken>
I have provided sample XML and want to know how to get the values the are not in the courses-taken but are in the courses-reqd by using XPATH. In the case provided, the answer would be Geog and Phys as they do not exist in course-taken.
<xsl:apply-templates select="my-courses\courses-reqd | not(my-courses\courses-taken)"/>
Is this possible and if so, how would I go abouts doing this. Any help would be appreciated.
Thanks

Answer 1

The simple answer here is to do this:
<xsl:apply-templates
select="/my-courses/courses-reqd/detail
[not(subject = /my-courses/courses/taken/details/subject)]" />
A better (more efficient) approach is to use keys.
This would be added towards the top of your XSLT:
<xsl:key name="kTaken" match="courses-taken//subject" use="."/>
And then this would be used in one of your templates:
<xsl:apply-templates
select="/my-courses/courses-reqd/detail[not(key('kTaken', subject))]" />
When this XSLT is run on your sample input (after the sample input has been corrected to consistently use elements named "details" and not "detail":
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" indent="yes"/>
<xsl:key name="kTaken" match="courses-taken//subject" use="."/>
<xsl:template match="/">
<root>
<xsl:apply-templates
select="/my-courses/courses-reqd/details[not(key('kTaken', subject))]" />
</root>
</xsl:template>
<xsl:template match="details">
<course>
<xsl:value-of select="subject"/>
</course>
</xsl:template>
</xsl:stylesheet>
The result is:
<root>
<course>Geog</course>
<course>Phys</course>
</root>