title: Challenging mysql INTERSECT query
body: <p>I am trying to find the mutual contacts between two users. The contacts table is set up like this:</p>
  
  <pre><code>userprofile_usercontact
  - first_user_id
  - second_user_id
  - mutual
  </code></pre>
  
  <p>The following is the query I am trying to achieve:</p>
  
  <pre><code>/* get the first user's contacts */
  SELECT second_user_id FROM userprofile_usercontact WHERE first_user_id = 1 AND mutual = 1
     UNION 
  SELECT first_user_id FROM userprofile_usercontact WHERE second_user_id = 1 AND mutual = 1
      *INTERSECT*
  /* get the second user's contacts */
  SELECT second_user_id FROM userprofile_usercontact WHERE first_user_id = 37 AND mutual = 1
      UNION 
  SELECT first_user_id FROM userprofile_usercontact WHERE second_user_id = 37 AND mutual = 1
  </code></pre>
  
  <p>How would I achieve this?</p>