Oracle Loop 循环的一个赋值坑

PROCEDURE test
 
IS
   comp_return_rec_     m_purchase_comp_return_tab%Rowtype;
			barcode_hist_rec_    m_barcode_inv_tran_hist_Tab%ROWTYPE;
			
			--
   CURSOR get_comp_return IS
				SELECT *
				FROM m_purchase_comp_return_tab m;
				
			--
			CURSOR get_barcode_hist IS
      SELECT m.* 
						FROM m_barcode_inv_tran_hist_Tab m , m_barcode_status_tab n
						WHERE n.barcode_no = m.barcode_no
						AND   n.co_order_no = comp_return_rec_.bill_no
						AND   n.co_line_no = comp_return_rec_.item_seq_no
						AND   n.source = "采购订单";
BEGIN 
   OPEN get_comp_return;
				LOOP
				 FETCH get_comp_return INTO comp_return_rec_ ;
					EXIT WHEN get_comp_return%NOTFOUND;
					
					barcode_hist_rec_.barcode_no := null;
					
					OPEN get_barcode_hist;
					FETCH get_barcode_hist INTO barcode_hist_rec_;
					CLOSE get_barcode_hist;
					
					IF barcode_hist_rec_.barcode_no IS NOT NULL THEN 
					   UPDATE m_purchase_comp_return_tab m SET m.barcode_created_flag = 2
								       WHERE m.bill_no = comp_return_rec_.bill_no
															AND   m.item_seq_no = comp_return_rec_.item_seq_no ;
					END IF ;
					
				END LOOP;
			CLOSE get_comp_return;
 
END test;

loop 的话空值时不会赋值的