$collection->addAttributeToFilter('somefield', 'null')

$collection->addAttributeToFilter('somefield', array('is' => 'null'))

$collection->addAttributeToFilter('somefield', array('null'=>'null')

$collection = Mage::getResourceModel('sales/order_shipment_item_collection');

$collection->addFieldToSelect('section_id');

$collection->addFieldToSelect('shelf_id');

$collection->addFieldToSelect('rack_id');

$collection->addFieldToSelect('box_id');

//$collection->addFieldToSelect('GROUP_CONCAT(DISTINCT main_table.product_id SEPARATOR ",")','product_ids');</p>

$collection->getSelect()->joinLeft('sales_flat_shipment_grid', 'main_table.parent_id = sales_flat_shipment_grid.entity_id', array('increment_id as shipment_increment_id', 'created_at as shipment_created_at', 'order_increment_id','total_qty'));

$collection->getSelect()->joinLeft('sales_flat_shipment_track', 'sales_flat_shipment_grid.entity_id = sales_flat_shipment_track.parent_id', array('number as track_id', 'carrier_code as carrier_name'));

$collection->getSelect()->joinLeft('sales_flat_order', 'sales_flat_shipment_track.order_id = sales_flat_order.entity_id', array('created_at as order_created_at','grand_total as ord_grand_total'));

$collection->getSelect()->joinLeft('sales_flat_order_item', 'main_table.order_item_id = sales_flat_order_item.item_id', array('product_type'));

$collection->getSelect()->joinLeft('sales_flat_order_payment', 'sales_flat_shipment_track.order_id = sales_flat_order_payment.parent_id', array('method as payment_method'));

$collection->getSelect()->joinLeft('sales_flat_order_address', 'sales_flat_shipment_track.order_id = sales_flat_order_address.parent_id', array('firstname','lastname','street','city','postcode','region','telephone'));

$collection->getSelect()->joinLeft('sales_flat_invoice', 'sales_flat_shipment_track.order_id = sales_flat_invoice.order_id', array('grand_total'));

//$collection->addAttributeToFilter('main_table.price', array('gt' => 0));

if($pickup_ids!='' && is_array($pickup_ids))

    $collection->addAttributeToFilter('sales_flat_shipment_grid.entity_id', array('in' => $pickup_ids));

$collection->getSelect()->columns(

    array('product_ids' => new Zend_Db_Expr(

        "IFNULL(GROUP_CONCAT(DISTINCT main_table.product_id SEPARATOR ';'), '')"

)));

$collection->getSelect()->columns(

    array('weight' => new Zend_Db_Expr(

        "IFNULL(SUM(main_table.weight)/2, '')"

)));

$collection->getSelect()->group('shipment_increment_id');

//$collection->getSelect()->group('track_id');

//Mage::log($collection->getSelect()->__toString());

$this->setCollection($collection);

//$this->getCollection()->getSelect()->limit();