对于两张表以上关联起来的视图，Oracle是分不出来应该对哪个表进行更新
而INSTEAD OF 触发器实现了我们对视图DML的需求，显示的告诉告诉Oracle要更新哪个表
和其他类型触发器不同的是，INSTEAD OF 触发器实际上并不是由某个事件触发的

语法：

CREATE OR REPLACE TRIGGER trigger_name
INSTEAD OF operation    --delete,update,insert,create and so on
ON view_name
FOR EACH ROW
BEGIN
  ...
END;

因为对TRIGGER印象不太好，从未主动想去亲近她
这里Think想强调的一点是，9i的文档有提到：
"INSTEAD OF triggers defined in the view are dropped when a view is re-created"
一贯以来的思维方式就是：如果修改了底层对象，那么重新编译上层的视图、触发器、存储过程等即可
只要状态为 VALID 就视为没有问题
然而，INSTEAD OF 触发器却无异于当头棒喝，原来Oracle还会有这样的脾气！
下面用实验来证明这句话

hr@ORCL> CREATE TABLE A (ID NUMBER(5),COL VARCHAR2(5));

Table created.

hr@ORCL> CREATE TABLE B (ID NUMBER(5),COL VARCHAR2(5));

Table created.

hr@ORCL> CREATE OR REPLACE VIEW V_AB AS
  2      SELECT A.ID,A.COL AS COLA,B.COL AS COLB
  3       FROM A,B
  4      WHERE A.ID=B.ID
  5      /

View created.

hr@ORCL> CREATE OR REPLACE TRIGGER TRG_V_AB
  2      INSTEAD OF INSERT ON V_AB
  3      FOR EACH ROW
  4      BEGIN
  5      INSERT INTO A(ID,COL)VALUES(:NEW.ID,:NEW.COLA);
  6      INSERT INTO B(ID,COL)VALUES(:NEW.ID,:NEW.COLB);
  7      END;
  8      /

Trigger created.

hr@ORCL> SELECT TRIGGER_NAME,TRIGGER_TYPE FROM USER_TRIGGERS;

TRIGGER_NAME                   TRIGGER_TYPE
------------------------------ ----------------
TRG_V_AB                       INSTEAD OF

      
hr@ORCL> INSERT INTO V_AB VALUES(1,\'AA\',\'BB\');

1 row created.

hr@ORCL> COMMIT;

Commit complete.

hr@ORCL> select * from a;

        ID COL
---------- -----
         1 AA

hr@ORCL> select * from b;

        ID COL
---------- -----
         1 BB

hr@ORCL> CREATE OR REPLACE VIEW  V_AB AS
  2      SELECT A.ID,B.COL AS COLA,A.COL AS COLB
  3       FROM A,B
  4      WHERE A.ID=B.ID
  5      /

View created.

hr@ORCL> INSERT INTO V_AB VALUES(1,\'AA\',\'BB\');
INSERT INTO V_AB VALUES(1,\'AA\',\'BB\')
*
ERROR at line 1:
ORA-01779: cannot modify a column which maps to a non key-preserved table


hr@ORCL> SELECT TRIGGER_NAME,TRIGGER_TYPE FROM USER_TRIGGERS;

no rows selected.

注释：
触发器，而且是行级触发器
:OLD：指更新以前的字段值
:new：指更新以后的字段值

ITPub博客

INSTEAD OF 触发器的使用陷阱

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