title: Delete all but one duplicate record
body: <p>I have a table that is supposed to keep a trace of visitors to a given profile (user id to user id pair). It turns out my SQL query was a bit off and is producing multiple pairs instead of single ones as intended. With hindsight I should have enforced a unique constraint on each id+id pair.</p>
  
  <p>Now, how could I go about cleaning up the table? What I want to do is delete all duplicate pairs and leave just one.</p>
  
  <p>So for example change this:</p>
  
  <pre><code>23515 -&gt; 52525 date_visited
  23515 -&gt; 52525 date_visited
  23515 -&gt; 52525 date_visited
  12345 -&gt; 54321 date_visited
  12345 -&gt; 54321 date_visited
  12345 -&gt; 54321 date_visited
  12345 -&gt; 54321 date_visited
  23515 -&gt; 52525 date_visited
  ...
  </code></pre>
  
  <p>Into this:</p>
  
  <pre><code>23515 -&gt; 52525 date_visited
  12345 -&gt; 54321 date_visited
  </code></pre>
  
  <p>Update: Here is the table structure as requested:</p>
  
  <pre><code>id  int(10)         UNSIGNED    Non     Aucun   AUTO_INCREMENT
  profile_id  int(10)         UNSIGNED    Non     0 
  visitor_id  int(10)         UNSIGNED    Non     0
  date_visited    timestamp           Non     CURRENT_TIMESTAMP   
  </code></pre>