title: MySQL select unique pairs
body: <p>My table looks like:</p>
  <pre><code>id A  B
  1  1  3
  2  2  3
  3  3  1
  </code></pre>
  <p>I want SELECT only unique AB pairs</p>
  <p>13 is unique</p>
  <p>23 is unique</p>
  <p>31 not unique because pair 13 exists</p>
  <p>Trying with <code>SELECT a, b FROM test GROUP BY a, b</code> but query return 3 pairs</p>