INSTEAD OF 触发器的使用陷阱

       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_nameINSTEAD OF operation    --delete,update,insert,create and so onON view_nameFOR EACH ROWBEGIN  ...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 AAhr@ORCL> select * from b;        ID COL---------- -----         1 BBhr@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 tablehr@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