title: Add and show new column in sales_flat_order and show this at order grid in magento
body: <p>I have added a new column (exported)in sales_flat_order and add at files at this location:</p>
  
  <p>app/code/core/Mage/Adminhtml/Block/Sales/Order/Grid.php</p>
  
  <pre><code>protected function _prepareCollection()
  {
   $collection = Mage::getResourceModel($this-&gt;_getCollectionClass())
      -&gt;addAttributeToSelect('*')
      -&gt;joinAttribute('exported','sales/order','sales_flat_order.entity_id',null,'left');
  }
  
  protected function _prepareColumns()
  {
   $this-&gt;addColumn('exported', array(
              'header' =&gt; Mage::helper('sales')-&gt;__('Exported'),
              'index' =&gt; 'exported',
              'type'  =&gt; 'checkbox',
              'name'  =&gt;'exported',
              'value'    =&gt;$this-&gt;getExported()==1 ? 'true' : 'false',
            ));
    }
  </code></pre>
  
  <p>after that it showing on order grid in admin site,but it is not showing value and name,</p>
  
  <p>I am new in magento,so please help me ,
  stuck from 2 days.
  Thanks for Assistance.</p>