title: unique key for two sides? avoid (1,2) (2,1) also
body: <p>I have a table conversations, I'd like to have only one conversation for each pair of users:</p>
  
  <p><code>to</code> - <code>from</code></p>
  
  <p>if I have user 1 (to) and user 2 (from) I would not like to allow user 2 (to) and user 1 (from) also.</p>
  
  <p>I have unique key:</p>
  
  <pre><code>UNIQUE KEY (`to`, `from`),
  </code></pre>
  
  <p>but it only works for one side (1,2) if I have (2,1) it will insert too.</p>
  
  <p>Any ideas how to make this unique key for both sides?</p>