oracle sql中的pivot时出现无效标识符错误

我试图从 order_tbl 调整销售额（即数量*价格），但得到

“ORA-00904：“销售”无效标识符”。

任何人都可以帮助以下sql有什么问题吗？

SELECT order_pay, product_id, (quantity * price) as sales
          FROM order_tbl
       PIVOT (SUM (sales) FOR order_pay IN ('01-MAY-2015', '02-MAY-2015'))

创建表和插入数据的脚本如下。

CREATE TABLE ORDER_TBL
(
  ORDER_PAY   DATE,
  ORDER_ID    VARCHAR2(10 BYTE),
  PRODUCT_ID  VARCHAR2(10 BYTE),
  QUANTITY    NUMBER(5),
  PRICE       NUMBER(5)
)

Insert into ORDER_TBL
   (ORDER_PAY, ORDER_ID, PRODUCT_ID, QUANTITY, PRICE)
 Values
   (TO_DATE('5/1/2015', 'MM/DD/YYYY'), 'ORD1', 'PROD1', 5, 5);
Insert into ORDER_TBL
   (ORDER_PAY, ORDER_ID, PRODUCT_ID, QUANTITY, PRICE)
 Values
   (TO_DATE('5/1/2015', 'MM/DD/YYYY'), 'ORD2', 'PROD2', 2, 10);
Insert into ORDER_TBL
   (ORDER_PAY, ORDER_ID, PRODUCT_ID, QUANTITY, PRICE)
 Values
   (TO_DATE('5/1/2015', 'MM/DD/YYYY'), 'ORD3', 'PROD3', 10, 25);
Insert into ORDER_TBL
   (ORDER_PAY, ORDER_ID, PRODUCT_ID, QUANTITY, PRICE)
 Values
   (TO_DATE('5/1/2015', 'MM/DD/YYYY'), 'ORD4', 'PROD1', 20, 5);
Insert into ORDER_TBL
   (ORDER_PAY, ORDER_ID, PRODUCT_ID, QUANTITY, PRICE)
 Values
   (TO_DATE('5/2/2015', 'MM/DD/YYYY'), 'ORD5', 'PROD3', 5, 25);
Insert into ORDER_TBL
   (ORDER_PAY, ORDER_ID, PRODUCT_ID, QUANTITY, PRICE)
 Values
   (TO_DATE('5/2/2015', 'MM/DD/YYYY'), 'ORD7', 'PROD1', 2, 5);
Insert into ORDER_TBL
   (ORDER_PAY, ORDER_ID, PRODUCT_ID, QUANTITY, PRICE)
 Values
   (TO_DATE('5/2/2015', 'MM/DD/YYYY'), 'ORD8', 'PROD5', 1, 50);
Insert into ORDER_TBL
   (ORDER_PAY, ORDER_ID, PRODUCT_ID, QUANTITY, PRICE)
 Values
   (TO_DATE('5/2/2015', 'MM/DD/YYYY'), 'ORD9', 'PROD6', 2, 50);
Insert into ORDER_TBL
   (ORDER_PAY, ORDER_ID, PRODUCT_ID, QUANTITY, PRICE)
 Values
   (TO_DATE('5/2/2015', 'MM/DD/YYYY'), 'ORD10', 'PROD2', 4, 10);
Insert into ORDER_TBL
   (ORDER_PAY, ORDER_ID, PRODUCT_ID, QUANTITY, PRICE)
 Values
   (TO_DATE('5/2/2015', 'MM/DD/YYYY'), 'ORD6', 'PROD4', 6, 20);
COMMIT;