Oracle：無法更改此觸發器類的New值

今天做ERP維護和管理工作中遇到一個問題，當一個值發生變化時，指定內容的值需要發生變化，

即

1、若該值由0變成1時，系統不做任何改變；

2、若該值由1變成0時，系統根據實際需求，刪除或清空該類的小項內容。

如圖：

結合該需求，筆者了解了觸發器的原理和用途，選擇運用觸發器原理來解決這一需求。

一開始，筆者未了解清楚:new的使用范疇，因此在書寫觸發器時將其寫在after之后執行，編譯時報出了“無法更改此觸發器類的New值”異常，尋思未解，於是開始尋找原因。之后才知道After時:new值只有讀的權限，沒有寫的權限，而Before時：new有讀寫權限，因此將After改為Before，編譯成功！

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;