title: How to select and exclude records from the same subset in LINQ
body: <p>I have a sample connection table <code>PolicyToX</code> with fields <code>Id, PolicyId, PersonId, SchoolId</code>. Records are always saved with one of the FKs being NULL, for example <code>1, 1, 5, NULL</code>.</p>
  
  <p>I want to write a query in LINQ that, when given two parameters: <code>PersonId</code> and <code>SchoolId</code> will filter all Policies of the given School but without those that are already bound to a given Person.</p>
  
  <p>So, if I have a dataset of:</p>
  
  <pre><code>[Id][PolicyId][PersonId][SchoolId]
   1     1         5        NULL
   2     1        NULL       1
   3     2        NULL       1
  </code></pre>
  
  <p>and pass paremeters <code>PersonId = 5</code> and <code>SchoolId = 1</code> the result should be one Policy of ID = 2.</p>
  
  <p>Thanks!</p>