title: use set to subtract the column value in update query
body: <p>I am working on a database column value (int) to subtract and update using <code>set</code>.</p>
  
  <p>I am using <code>$this-&gt;db-&gt;set()</code> to subtract and update column value in codeigniter. </p>
  
  <p>The problem is when I run the query the column value get change to null. </p>
  
  <pre><code>$this-&gt;db-&gt;set('app_balance', 'app_balance'. - $app_work_charge.'');
  $this-&gt;db-&gt;where('id', $app_id);
  $this-&gt;db-&gt;update('app');
  </code></pre>
  
  <p>In my log file it will not showing any error.</p>
  
  <p>More Explanation: if column value is 200 and if I will want to subtract 10 from it then after query execution it will change to <code>null</code>. </p>