protected function _renderFiltersBefore() {
  $joinTable = $this->getTable('sales_order');
  $this->getSelect()->joinLeft($joinTable,
       'main_table.entity_id = sales_order.entity_id', ['coupon_code']);
  $joinTableItem = $this->getTable('sales_order_item');
  $this->getSelect()->join($joinTableItem,
       'main_table.entity_id = sales_order_item.order_id',
       ['sku','price','qty_ordered']);
  parent::_renderFiltersBefore();
}