title: GROUP BY - do not group NULL
body: <p>I'm trying to figure out a way to return results by using the group by function.</p>
  <p>GROUP BY is working as expected, but my question is: Is it possible to have a group by ignoring the NULL field. So that it does not group NULLs together because I still need all the rows where the specified field is NULL.</p>
  <pre><code>SELECT `table1`.*, 
      GROUP_CONCAT(id SEPARATOR ',') AS `children_ids`
  FROM `table1` 
  WHERE (enabled = 1) 
  GROUP BY `ancestor` 
  </code></pre>
  <p>So now let's say I have 5 rows and the ancestor field is NULL, it returns me 1 row....but I want all 5.</p>