title: Adding 'CASE WHEN' to codeigniter CI_DB_mysql_driver object's ar_select key
body: <p>I need to add a case when statement to CI_DB_mysql_driver object. I tried the following code</p>
  
  <pre><code>$this-&gt;db-&gt;select("CASE WHEN colum1 &gt;0 THEN 'sl' ELSE colum1 END AS colum1");
  </code></pre>
  
  <p>When I run it query is generated as</p>
  
  <pre><code>`CASE` WHEN colum1 &gt;0 THEN 'sl' ELSE colum1 END AS colum1
  </code></pre>
  
  <p>and it generates sql error.</p>