title: Strange GROUP_CONCAT behaviour in MySQL
body: <p>I ran into a really strange problem today when using the MySQL function <code>GROUP_CONCAT</code>:</p>
  
  <p>I have the following query:</p>
  
  <pre><code>SELECT SUM(total) FROM order WHERE customer_id='X' AND order_status_id IN ((SELECT GROUP_CONCAT(order_status_id SEPARATOR ',') FROM order_status WHERE profit='1'))
  </code></pre>
  
  <p>but that returns NULLL, however:</p>
  
  <pre><code>SELECT SUM(total) FROM order WHERE customer_id='X' AND order_status_id IN (1,2,3,4,5,6,7,8)
  </code></pre>
  
  <p>this works as well as the first query to concat the status id's, grouped however they return NULL as total</p>