title: yii framework, how to implement union sql
body: <p>i am trying to do something like this on yii actionAdmin()</p>
  
  <pre><code>select * from car where carbrand = 'bmw' limit 1
  union
  select * from car where carbrand = 'mercedez'
  </code></pre>
  
  <p>i cannot find anything to do with union on CDbCriteria</p>
  
  <p>Is there any other way?</p>