title: Query between two related tables
body: <p>I have to tables. One of them, <code>user_profile</code>, has a field named <code>user_profile_id</code> and the other table, <code>user_friend</code>, has two fields named <code>user_profile1_id</code> and <code>user_profile2_id</code> (which are FK to the first table). </p>
  
  <p>I want to check if there is any users in user_profile which is not in user_friend table, neither in user_profile1_id, nor user_profile2_id. what query should I use !?</p>