Recently, I have worked on magento admin grid and I want to add custom column with filter on customer grid without adding column in customer table. I have faced lot of issue during this development and after digging continuously 2 hour I found terms
addExpressionAttributeToSelect . This is finally solved my problem.
I did customization on
_prepareCollection function and _
prepareColumns function .It worked fine.
prepareCollection function, add below expression after getting the $collection
$collection->addExpressionAttributeToSelect('column_column', new Zend_Db_Expr("CUSTOM_MYSQL_EXPRESSSION"), array());
And add following column code in
'header' => Mage::helper('customer')->__('CUSTOM COLUMN'),
'index' => 'custom_column',