title: Group by date mysql and codeigniter
body: <p>I have table like this:</p>
  
  <pre><code>date_time               value
  2015-06-20 10:00:00     10
  2015-06-20 9:00:00      6
  2015-06-21 6:00:00      5
  2015-06-22 11:00:00     12
  2015-06-22 10:00:00     11
  2015-06-23 4:00:00      20
  </code></pre>
  
  <p>I want to get the total value of each date
  Currently I have this on CodeIgniter but is not working:</p>
  
  <pre><code>$this-&gt;db-&gt;group_by('date_time');
  $query = $this-&gt;db-&gt;get('table_a');
  </code></pre>
  
  <p>Any idea to fix. Thanks.</p>