title: Possible to check if a number is IN a GROUP_CONCAT
body: <p>I want to only get those ids where there are 2 rows and one row has the <code>site_id = 1</code>, the other <code>site_id = 2</code>.  </p>
  
  <p>I tried joining the table on twice, but the query took too long.  So what I am now doing is concatenating the site ids, and restricting the rows to only those where 1 is in this concat value, and 28 is in it.  However, I can't get it to work.  It is to do with site_id being a number and my concat values i.e. <code>sites</code> being a string?  How can I make sure that both these values are in my concatenated string?</p>
  
  <pre><code>SELECT *, COUNT(id) as num_ids
  FROM (
      SELECT id, CONVERT(GROUP_CONCAT(`site_id`) USING utf8) as `sites`, MAX(`Date_Added`) as `date_added_lb`
      FROM `product_location` pl
      WHERE `site_id` =1 OR `site_id` = 28
      GROUP BY id
  ) as t
  WHERE t.`date_added_lb` &gt;="2013-02-27"
  AND 1 IN(`sites`)
  AND 28 IN(`sites`)
  AND num_ids=2
  </code></pre>