好，接下来我们来做测试，先登录到oe用户，查相关的表。
[html]
gyj@OCM> conn oe/oe
Connected.
oe@OCM> select table_name from tabs;
TABLE_NAME
------------------------------
PRODUCT_REF_LIST_NESTEDTAB
SUBCATEGORY_REF_LIST_NESTEDTAB
PROMOTIONS
ORDERS
PRODUCT_DESCRIPTIONS
WAREHOUSES
PRODUCT_INFORMATION
ORDER_ITEMS
CUSTOMERS
INVENTORIES
10 rows selected.
gyj@OCM> conn oe/oe
Connected.
oe@OCM> select table_name from tabs;
TABLE_NAME
------------------------------
PRODUCT_REF_LIST_NESTEDTAB
SUBCATEGORY_REF_LIST_NESTEDTAB
PROMOTIONS
ORDERS
PRODUCT_DESCRIPTIONS
WAREHOUSES
PRODUCT_INFORMATION
ORDER_ITEMS
CUSTOMERS
INVENTORIES
10 rows selected.
一、答案A，很明显是错的，视图的字段与表的字段的个数不一样，操作如下报错：
[html]
oe@OCM> CREATE OR REPLACE VIEW ord_vu(order_id,order_date)
2 AS SELECT o.order_id,o.order_date,COUNT(i.line_item_id) "NO OF ITEMS" FROM orders o JOIN order_items i ON(o.order_id=i.order_id)
3 GROUP BY o.order_id,o.order_date;
CREATE OR REPLACE VIEW ord_vu(order_id,order_date)
*
ERROR at line 1:
ORA-01730: invalid number of column names specified
在CREATE OR REPLACE VIEW ord_vu(order_id,order_date,ct)添加一列CT,操作如下就没问题：
oe@OCM> CREATE OR REPLACE VIEW ord_vu(order_id,order_date,ct)
2 AS SELECT o.order_id,o.order_date,COUNT(i.line_item_id) "NO OF ITEMS" FROM orders o JOIN order_items i ON(o.order_id=i.order_id)
3 GROUP BY o.order_id,o.order_date;
View created.
oe@OCM> CREATE OR REPLACE VIEW ord_vu(order_id,order_date)
2 AS SELECT o.order_id,o.order_date,COUNT(i.line_item_id) "NO OF ITEMS" FROM orders o JOIN order_items i ON(o.order_id=i.order_id)
3 GROUP BY o.order_id,o.order_date;
CREATE OR REPLACE VIEW ord_vu(order_id,order_date)
*
ERROR at line 1:
ORA-01730: invalid number of column names specified
在CREATE OR REPLACE VIEW ord_vu(order_id,order_date,ct)添加一列CT,操作如下就没问题：
oe@OCM> CREATE OR REPLACE VIEW ord_vu(order_id,order_date,ct)
2 AS SELECT o.order_id,o.order_date,COUNT(i.line_item_id) "NO OF ITEMS" FROM orders o JOIN order_items i ON(o.order_id=i.order_id)
3 GROUP BY o.order_id,o.order_date;
View created.
二、答案B是正确的，把视图定义的列名去掉，视图默认这些列名来自select中的显示的列，操作如下：
[html]
oe@OCM> CREATE OR REPLACE VIEW ord_vu
2 AS SELECT o.order_id,o.order_date,COUNT(i.line_item_id) "NO OF ITEMS" FROM orders o JOIN order_items i ON(o.order_id=i.order_id)
3 GROUP BY o.order_id,o.order_date;
View created.
oe@OCM> select * from ord_vu;
ORDER_ID ORDER_DATE NO OF ITEMS
---------- --------------------------------------------------------------------------- -----------
2354 15-JUL-08 08.18.23.234567 AM 13
2361 14-NOV-07 05.34.21.986210 AM 9
2363 24-OCT-07 07.49.56.346122 AM 9
2367 28-JUN-08 11.53.32.335522 AM 8
省略结果。。。。。。。。。。。。。。。
oe@OCM> CREATE OR REPLACE VIEW ord_vu
2 AS SELECT o.order_id,o.order_date,COUNT(i.line_item_id) "NO OF ITEMS" FROM orders o JOIN order_items i ON(o.order_id=i.order_id)
3 GROUP BY o.order_id,o.order_date;
Vi