如何通過觸發器本身刪除關於哪個觸發器被調用的行？

Question

我的問題如下， 首先這裏是我的代碼，

create or replace trigger tri_ISSUE 
after insert or delete 
on ISSUE 
for each row 
declare 
    ct varchar2(20); 
    cnt number(20); 
begin 
    if inserting 
    then 
     select bk_category into ct from BOOK_MASTER where accn_no=:new.accn_no; 
     select available into cnt from BOOK_INFO where bk_category=ct; 
     if(cnt=0) 
     then 
      dbms_output.put_line('Book is not available!'); 
      delete ISSUE where rollno=:new.rollno and accn_no=:new.accn_no; 
     else 
      select bk_category into ct from BOOK_MASTER where accn_no=:new.accn_no; 
      update BOOK_INFO set available=available-1 where bk_category=ct; 
     end if; 
    end if; 
    if deleting 
    then 
     select bk_category into ct from BOOK_MASTER where accn_no=:old.accn_no; 
     update BOOK_INFO set available=available+1 where bk_category=ct; 
    end if; 
end; 

我得到這樣的錯誤，

ORA-04091: table SYSTEM.ISSUE is mutating, trigger/function may not see it 
ORA-06512: at "SYSTEM.TRI_ISSUE", line 12 
ORA-04088: error during execution of trigger 'SYSTEM.TRI_ISSUE' 

什麼，我試圖做的是，我要刪除通過插入後觸發的'After Insert'觸發器插入正在插入的行。我可以這樣做嗎？任何人都可以建議我補救嗎？在此先感謝:)

Answer 1

有幾種方法來實現您的要求。

1. 把你所有的邏輯放到一個過程中。這很可能是最好的做法。

2. 通過raise_application_error(-20001, 'Book is not available!');

   更換線delete ISSUE where rollno = :new.rollno and accn_no = :new.accn_no;利用該用戶得到一個異常並且數據不被插入。

3. 創建一個compound trigger與BEFORE STATEMENT,BEFORE EACH ROW和AFTER STATEMENT組件。

4. 創建一個視圖和INSTEAD OF TRIGGER，會是這樣：

   CREATE OR REPLACE VIEW V_ISSUE AS 
   SELECT * FROM ISSUE; 
   
   CREATE OR REPLACE TRIGGER tri_ISSUE 
       INSTEAD OF INSERT OR DELETE ON V_ISSUE 
       FOR EACH ROW 
   
   declare 
       ct varchar2(20); 
       cnt number(20); 
   begin 
       if inserting 
       then 
        select bk_category into ct from BOOK_MASTER where accn_no=:new.accn_no; 
        select available into cnt from BOOK_INFO where bk_category=ct; 
        if(cnt=0) 
        then 
         dbms_output.put_line('Book is not available!'); 
         NULL; -- > do nothing 
        else 
         INSERT INTO ISSUE VALUES (... all coloums of this table); 
         select bk_category into ct from BOOK_MASTER where accn_no=:new.accn_no; 
         update BOOK_INFO set available=available-1 where bk_category=ct; 
        end if; 
       end if; 
       if deleting 
       then 
        DELETE FROM ISSUE WHERE ...; 
        select bk_category into ct from BOOK_MASTER where accn_no=:old.accn_no; 
        update BOOK_INFO set available=available+1 where bk_category=ct; 
       end if; 
   END;