Oracle：无法更改此触发器类的New值

今天做ERP维护和管理工作中遇到一个问题，当一个值发生变化时，指定内容的值需要发生变化，

即

1、若该值由0变成1时，系统不做任何改变；

2、若该值由1变成0时，系统根据实际需求，删除或清空该类的小项内容。

如图：

结合该需求，笔者了解了触发器的原理和用途，选择运用触发器原理来解决这一需求。

一开始，笔者未了解清楚:new的使用范畴，因此在书写触发器时将其写在after之后执行，编译时报出了“无法更改此触发器类的New值”异常，寻思未解，于是开始寻找原因。之后才知道After时:new值只有读的权限，没有写的权限，而Before时：new有读写权限，因此将After改为Before，编译成功！

SQL如下：

修改前：

create or replace trigger tri_app1_mstr_upd after update on app1_mstr
for each row
begin
  if updating then
    if :new.app_hardware = 0 and :old.app_hardware = 1 then
      :new.app_1_dept      := '';
      :new.app_1_mgr1      := '';
      :new.app_1_summary   := '';
      :new.app_1_engineer1 := '';
      :new.app_1_engineer2 := '';
      :new.app_1_engineer3 := '';
      
      delete from appd6_det where appd6_flow = :old.app_flow;
    end if;        
  end if; 
end tri_app1_mstr_upd;

修改后：

create or replace trigger tri_app1_mstr_upd before update on app1_mstr
for each row
begin
  if updating then
    if :new.app_hardware = 0 and :old.app_hardware = 1 then
      :new.app_1_dept      := '';
      :new.app_1_mgr1      := '';
      :new.app_1_summary   := '';
      :new.app_1_engineer1 := '';
      :new.app_1_engineer2 := '';
      :new.app_1_engineer3 := '';
      
      delete from appd6_det where appd6_flow = :old.app_flow;
    end if;
    
    if :new.app_structure = 0 and :old.app_structure = 1 then
      :new.app_2_dept       :='';
      :new.app_2_mgr1       :='';
      :new.app_2_summary    :='';
      :new.app_2_engineer1  :='';
      :new.app_2_engineer2  :='';
      :new.app_2_engineer3  :='';
      
      delete from appd7_det where appd7_flow = :old.app_flow;
    end if;
    
    if :new.app_software = 0 and :old.app_software = :old.app_flow then
      :new.app_3_dept       :='';
      :new.app_3_mgr1       :='';
      :new.app_3_summary    :='';
      :new.app_3_engineer1  :='';
      :new.app_3_engineer2  :='';
      :new.app_3_engineer3  :='';
      
      delete from appd8_det where appd8_flow = :old.app_flow;
    end if;
    
    if :new.app_other = 0 and :old.app_other = 1 then
      :new.app_4_dept       :='';
      :new.app_4_mgr1       :='';
      :new.app_4_summary    :='';
      :new.app_4_engineer1  :='';
      :new.app_4_engineer2  :='';
      :new.app_4_engineer3  :='';
      
      delete from appd9_det where appd9_flow = :old.app_flow;
    end if;
    
    if :new.app_module = 0 and :old.app_module = 1 then
      :new.app_5_dept       :='';
      :new.app_5_mgr1       :='';
      :new.app_5_summary    :='';
      :new.app_5_engineer1  :='';
      :new.app_5_engineer2  :='';
      :new.app_5_engineer3  :='';
      
      delete from appd12_det where appd12_flow = :old.app_flow;
    end if;    
  end if; 
end tri_app1_mstr_upd;