[每日一题] OCP1z0-047 :2013-07-29 视图――别名

本题的考点是如何创建视图，对于视图的详细知识点，可以参考我的博客：

http://blog.csdn.net/guoyjoe/article/details/8614677

好，接下来我们来做测试，先登录到oe用户，查相关的表。

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：

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：

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。。。。。。。。。。。。。。。

三、答案C是错的，在创建视图时，对这种使用各种函数，或运算表达式的列，一定要起别名，如没有别名视图创建就会失败，操作如下：

oe@OCM> CREATE OR REPLACE VIEW ord_vu 
  2  AS SELECT o.order_id,o.order_date,COUNT(i.line_item_id) FROM orders o JOIN order_items i ON(o.order_id=i.order_id)
  3  GROUP BY o.order_id,o.order_date;
AS SELECT o.order_id,o.order_date,COUNT(i.line_item_id) FROM orders o JOIN order_items i ON(o.order_id=i.order_id)
                                  *
ERROR at line 2:
ORA-00998: must name this expression with a column alias

把上面的视图改成如下：
oe@OCM> CREATE OR REPLACE VIEW ord_vu 
  2  AS SELECT o.order_id,o.order_date,COUNT(i.line_item_id)  CT 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.

四、答案D也是错的，错误与答案C一个问题

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
  4  WITH CHECK OPTION;
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)
                                                        *
ERROR at line 2:
ORA-00998: must name this expression with a column alias

 把上面的视图改成如下：
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' CT FROM orders o JOIN order_items i ON(o.order_id=i.order_id)
  3  GROUP BY o.order_id,o.order_date
  4  WITH CHECK OPTION;

View created.

正确答案：B

结总：

在创建视图时，对这种使用各种函数，或运算表达式的列，一定要起别名，如没有别名视图创建就会失败。

定义视图的列名可以省略，来自SELECT定义中的列名，如果定义视图的列名不省略，那个列的个数与SELECT定义中的列的个数要一致。