这两天就遇到一个问题，有两张表的一个字段需要进行同步更新，也就是A表修改时要把对应的B表的记录
字段修改，反过来B表修改时也要把A表的修改，保持两边数据的一个同步，这个可以在前台很容易的实现，但开发
人员不想修改代码了，就考虑在后台用trigger实现。
      功能很简单，但在实现时遇到一个问题，就是A上的DML触发了上面的TRIGGER，然后这个TRIGGER去更新B表，这样
就会触发B表上的触发器，而B表上的TRIGGER又会更新A表，这样就迭代触发，没有结束了，也就是会产生变异表（mutating）
我不知道ORACLE的触发器是否有属性来限制这种情况的发生，但以前做SQL SERVER时知道有种Instaed of的触发器，他表示
当DML启动他后，他将以TRIGGER里的代码来代替这个DML动作，也就是DML不会真正的执行，只会启动INSTEAD TRIGGER，最终
执行的是TRIGGER里面的编码。
      查看了Docs，看到ORACLE也支持这个类型的触发器，但这个只能建立到视图上，不能基于表建立，我要的功能肯定是可以
实现的，在这里我把原表进行了rename，引如了两张视图，名字就是以前的表名，这样对于他们前台应用就做了个
透明的切换，然后在两个视图上建立INSTEAD触发器，将任何两个视图上的更新都传播到后面的两个基表，这样不管你更新那个
视图，我都可以捕获到数据，以代码在后面更新，也不存在互相触发，因为触发器修改的对象已经转移到表了，而此时表上是没有
trigger的，呵呵！！！
过程如下

--创建测试表
SQL> create table mytest1(row_num number,row_name varchar2(50));

表被创建

SQL> create table mytest2(row_num number,row_name varchar2(50));

表被创建

--测试数据
SQL> INSERT INTO MYTEST1 VALUES(1,'Fuck!!!');

1 行 已插入

SQL> INSERT INTO MYTEST2 VALUES(1,'Watch your mouth!!!');

1 行 已插入

SQL> COMMIT;

提交完成

--先在一个表上创建触发器
SQL> CREATE OR REPLACE TRIGGER TRI_TEST1
  2  BEFORE UPDATE
  3  ON MYTEST1
  4  FOR EACH ROW
  5  DECLARE
  6  lv_new VARCHAR2(20);
  7  lv_parent VARCHAR2(20);
  8  BEGIN
  9      lv_new := :new.row_name;
 10      lv_parent := :OLD.row_name;
 11      IF lv_new <> lv_parent THEN
 12          UPDATE MYTEST2
 13          SET ROW_NAME = :NEW.ROW_NAME
 14          WHERE ROW_NUM = :NEW.ROW_NUM;
 15      END IF;
 16      DBMS_OUTPUT.PUT_LINE(lv_new || lv_parent);
 17  END;
 18  /

触发器被创建

--测试更新
SQL> set serveroutput on
SQL> UPDATE MYTEST1 SET ROW_NAME = 'DO it!!!';
DO it!!! Fuck!!!

1 行 已更新

--更新成功
SQL> SELECT * FROM MYTEST2;

   ROW_NUM ROW_NAME
---------- --------------------------------------------------
         1 DO it!!!
        
--另外张表创建触发器
SQL> CREATE OR REPLACE TRIGGER TRI_TEST2             
  2  BEFORE UPDATE                                   
  3  ON MYTEST2                                      
  4  FOR EACH ROW                                    
  5  DECLARE                                         
  6  lv_new VARCHAR2(20);                            
  7  lv_parent VARCHAR2(20);                         
  8  BEGIN                                           
  9      lv_new := :new.row_name;                       
 10      lv_parent := :OLD.row_name;                    
 11      IF lv_new <> lv_parent THEN                     
 12          UPDATE MYTEST1                               
 13          SET ROW_NAME = :NEW.ROW_NAME                 
 14          WHERE ROW_NUM = :NEW.ROW_NUM;                
 15      END IF;                                        
 16      DBMS_OUTPUT.PUT_LINE(lv_new || lv_parent);     
 17  END;                                            
 18  /  
 
--产生了变异表，更新失败
SQL> update mytest1 set row_name = 'mouthkkkkkoo';

update mytest1 set row_name = 'mouthkkkkkoo'

ORA-04091: table MYTEST1 is mutating, trigger/function may not see it
ORA-06512: at "TRI_TEST2", line 8
ORA-04088: error during execution of trigger 'TRI_TEST2'
ORA-06512: at "TRI_TEST1", line 8
ORA-04088: error during execution of trigger 'TRI_TEST1'   

--更新失败
SQL> update mytest2 set row_name = 'mouthkkkkkoo';

update mytest2 set row_name = 'mouthkkkkkoo'

ORA-04091: table MYTEST2 is mutating, trigger/function may not see it
ORA-06512: at "TRI_TEST1", line 8
ORA-04088: error during execution of trigger 'TRI_TEST1'
ORA-06512: at "TRI_TEST2", line 8
ORA-04088: error during execution of trigger 'TRI_TEST2' 

--删除触发器
SQL> drop trigger TRI_TEST2;

触发器被删掉

SQL> drop trigger TRI_TEST1;

触发器被删掉

--创建视图
SQL> CREATE VIEW V_TEST1 AS SELECT * FROM MYTEST1;

视图被创建

SQL> CREATE VIEW V_TEST2 AS SELECT * FROM MYTEST2;

视图被创建

--基于视图创建Instead触发器
SQL> CREATE OR REPLACE TRIGGER TRI_TEST1
  2  INSTEAD OF UPDATE
  3  ON V_TEST1
  4  FOR EACH ROW
  5  DECLARE
  6  lv_new VARCHAR2(20);
  7  lv_parent VARCHAR2(20);
  8  BEGIN
  9      lv_new := :new.row_name;
 10      lv_parent := :OLD.row_name;
 11      IF lv_new <> lv_parent THEN
 12          UPDATE MYTEST2
 13          SET ROW_NAME = :NEW.ROW_NAME
 14          WHERE ROW_NUM = :NEW.ROW_NUM;
 15          UPDATE MYTEST1
 16          SET ROW_NAME = :NEW.ROW_NAME
 17          WHERE ROW_NUM = :NEW.ROW_NUM;
 18      END IF;
 19      DBMS_OUTPUT.PUT_LINE(lv_new || lv_parent);
 20  END;
 21  /

触发器被创建

SQL> CREATE OR REPLACE TRIGGER TRI_TEST2
  2  INSTEAD OF UPDATE
  3  ON V_TEST2
  4  FOR EACH ROW
  5  DECLARE
  6  lv_new VARCHAR2(20);
  7  lv_parent VARCHAR2(20);
  8  BEGIN
  9      lv_new := :new.row_name;
 10      lv_parent := :OLD.row_name;
 11      IF lv_new <> lv_parent THEN
 12          UPDATE MYTEST2
 13          SET ROW_NAME = :NEW.ROW_NAME
 14          WHERE ROW_NUM = :NEW.ROW_NUM;
 15          UPDATE MYTEST1
 16          SET ROW_NAME = :NEW.ROW_NAME
 17          WHERE ROW_NUM = :NEW.ROW_NUM;
 18      END IF;
 19      DBMS_OUTPUT.PUT_LINE(lv_new || lv_parent);
 20  END;
 21  /

触发器被创建  

--功能已经实现
SQL> update v_test1 set row_name = 'I rock with you!!!';                                        
                                                            
1 行 已更新                                                 
                                                            
SQL> commit;                                                
                                                            
提交完成                                                    
                                                            
SQL> select * from v_test2;                                 
                                                            
   ROW_NUM ROW_NAME                                         
---------- --------------------------------------------------
         1 I rock with you!!!                                                                    
                                                            
                                                            
SQL> update v_test2 set row_name = 'Don''t kick me!!!';     
                                                            
1 行 已更新                                                 
                                                            
SQL> commit;                                                
                                                            
提交完成                                                    
                                                            
SQL> select * from v_test1;                                 
                                                            
   ROW_NUM ROW_NAME                                         
---------- --------------------------------------------------
         1 Don't kick me!!!                                 
                                                            
SQL>