title: SQL: Eliminate chains of duplicate in a GROUP_CONCAT
body: <p>I'm running a query to analyse variations in Group_member_ID for some users.
  What I would like to identify is the key variations in group_member_ids (eg Group1 on 01/08/2011, Group5 on 05/08/2011).</p>
  
  <p>I came up with this command:</p>
  
  <pre><code>select id, 
  CAST(group_concat(concat('[',group_member_id,'-',from_unixtime(obs_time),']') order by obs_time) as CHAR(10000) CHARACTER SET utf8) 
  from Table1 
  where id=1
  </code></pre>
  
  <p>RESULT:</p>
  
  <blockquote>
    <p>imei  group/date</p>
  </blockquote>
  
  <pre><code>1[178-2011-06-13 18:58:31],[0-2011-06-13 19:20:56],[0-2011-06-17 17:21:57],[0-2011-06-19 16:53:29],[0-2011-06-22 16:41:11],[178-2011-09-30 16:43:11],[179-2011-10-01 18:43:11]
  </code></pre>
  
  <p>How can I eliminate the Group/date <code>[0-2011-06-17 17:21:57],[0-2011-06-19 16:53:29],[0-2011-06-22 16:41:11]</code> from this query as I already identified the first record for group_member_id=0 and the others do not matter for me...
  In other words, I would like my final result to look like:</p>
  
  <blockquote>
    <p>imei  group/date</p>
  </blockquote>
  
  <pre><code>1[178-2011-06-13 18:58:31],[0-2011-06-13 19:20:56],[178-2011-09-30 16:43:11],[179-2011-10-01 18:43:11]
  </code></pre>
  
  <p>I'm stuck. I was thinking of using LIMIT in my group_concat but apparently it's not possible. Or is it?</p>
  
  <p>Thanks for your answers.</p>