title: select people who register the same course but dont know anyone
body: <p>Im having trouble find people from the same course I have 3 tables, <code>people(id, name)</code>, <code>knows(id,personA_id,personB_id)</code>, <code>course(id,person_id,course_id)</code> in knows table, person A knows person B if there is something in the row on <code>personB_id</code>.</p>
  
  <p>So I was thinking:</p>
  
  <pre><code>SELECT p.name FROM people p,knows k
  WHERE p.id = k.personA_id 
      AND p.id NOT IN(select k.personB_id from knows)
  </code></pre>
  
  <p>This would return me something that's relevant but I'm missing in the same classes.</p>