XML : Efficient way of finding elements that exists in one document but not the other in XQuery

I have the following data:

  <Subjects>      <Subject>          <Id>1</Id>          <Name>Maths</Name>      </Subject>      <Subject>          <Id>2</Id>          <Name>Science</Name>      </Subject>      <Subject>          <Id>2</Id>          <Name>Advanced Science</Name>      </Subject>      <Subject>          <Id>500</Id>          <Name>XYZ</Name>      </Subject>      <Subject>          <Id>1000</Id>          <Name>ABC</Name>      </Subject>  </Subjects>    

and:

  <Courses>      <Course>          <SubjectId>1</SubjectId>          <Name>Algebra I</Name>      </Course>      <Course>          <SubjectId>1</SubjectId>          <Name>Algebra II</Name>      </Course>      <Course>          <SubjectId>1</SubjectId>          <Name>Percentages</Name>      </Course>      <Course>          <SubjectId>2</SubjectId>          <Name>Physics</Name>      </Course>      <Course>          <SubjectId>2</SubjectId>          <Name>Biology</Name>      </Course>  </Courses>    

and I want to be able to get the subject elements with 500 and 1000, because they don't appear in the 2nd XML document.

How do I do this in the most efficient way possible (bare in mind I have about 750 subjects, each with 120 courses)?