[每日一题]OCP1z0-047:2013-07-29视图??别名好，接下来我们来做测试，先登录到oe用户，查相关的表。[html]gyj@OCM>connoe/oeConnected.oe@OCM>selecttable_namefromtabs;TABLE_NAME----...
[每日一题]OCP1z0-047 :2013-07-29视图??别名
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：
[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.
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。。。。。。。。。。。。。。。
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：
[html]
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
[html]
oe@OCM> CREATE OR REPLACE VIEW ord_vu
2 AS SELECT o.order_id,o.order_date,COUNT(i.line_item_id)||&＃39;NO OF ITEMS&＃39; 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)||&＃39;NO OF ITEMS&＃39; 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)||&＃39;NO OF ITEMS&＃39; 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。
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