Question

此新租借历史记录表的触发器，可防止从表中删除。

CREATE OR REPLACE  TRIGGER RENTALHIS_DEL
BEFORE DELETE ON RENTALHISTORY
BEGIN
dbms_output.put_line( 'Records can not be deleted');
END;

DELETE FROM RENTALHISTORY WHERE RENTALID = 1;



-- It is deleting before it says it can not delete

1 rows deleted.

Records can not be deleted

Answer 1

dbms_output.put_line( 'Records can not be deleted');

以上只是打印文本并触发成功完成，然后无论如何都会删除。你想做的是提出一个错误，以防止程序完成。

使用标准程序raise_application_error停止程序并引发错误：

CREATE OR REPLACE  TRIGGER RENTALHIS_DEL
BEFORE DELETE ON RENTALHISTORY
BEGIN
    raise_application_error(-20001,'Records can not be deleted');
END;
/

Answer 2

您想引发错误，而不是打印消息。无论如何都会发生删除：

CREATE OR REPLACE  TRIGGER RENTALHIS_DEL
BEFORE DELETE ON RENTALHISTORY
BEGIN
  RAISE_APPLICATION_ERROR (-20000, 'Deletion not supported on this table');
END;

或者，您可以使用instead of触发器来阻止删除：

CREATE OR REPLACE TRIGGER RENTALHIS_DEL
INSTEAD OF DELETE ON RENTALHISTORY
BEGIN
   dbms_output.put_line( 'Records cannot be deleted');
END;

Answer 3

之后，您可以使用commit after your delete statement and rollback来使用Pragma进入上一个阶段。

     CREATE OR REPLACE  TRIGGER 
     RENTALHIS_DEL
    AFTER DELETE ON RENTALHISTORY
       DECLARE
       PRAGMA 
          AUTONOMOUS_TRANSACTION;
      BEGIN
      RAISE_APPLICATION_ERROR 
     (-20000, 'Deletion getting rolled 
     back');
           ROLLBACK;
             END;

触发以防止从表中删除任何内容

3 个答案: