在触发器中使用更新

Question

我有删除触发器我想删除警察但是当我删除他时，所有其他他是他们老板的警察在他们的老板字段中都会有空所以我使用了这段代码

create or replace trigger switch_boss
before delete
on policeman 
for each row
declare
boss number; 
begin
boss := :new.bossid;
if(:new.policemanid = :new.bossid)then
select policemanid into boss from 
(select * from policeman
order by dbms_random.value)
where rownum =1;

end if;

update policeman
set bossid = boss
where bossid = :new.policemanid;

end switch_boss;

我收到错误

ORA-04091: table SYSTEM.POLICEMAN is mutating, trigger/function may not see it
ORA-06512: at "SYSTEM.SWITCH_BOSS", line 13
ORA-04088: error during execution of trigger 'SYSTEM.SWITCH_BOSS'

有什么想法吗？

更新：我使用 compund 触发它的工作，但不是我想要的。我想将被删除的警察的老板设置为他所属的老板的老板。问题是删除时我现在无法确定哪些警察将删除的警察作为老板。我能找到它们是因为它们在删除后的字段中为空，但它们可能属于其他已删除的警察。

这是我制作的触发器：

create or replace trigger switch_boss
  for delete
  on policeman 
compound trigger

  after statement is
  cursor c is select * from policeman where bossid is null for update;
  boss number;

  begin
    for r in c loop

      select policemanid into boss from 
                         (select * from policeman order by dbms_random.value)
                         where rownum =1;

      update policeman
      set bossid = boss
      where current of c;
   end loop;
  end after statement;

end switch_boss;

Answer 1

这是开始使用触发器时非常常见的问题。除了通过 :NEW 和 :OLD pseudo-records。这个想法是table处于不断变化的状态，所以它不能被触发器查询或修改..

处理此问题的典型方法是创建两个触发器：一个 row-level 触发器后跟一个 statement-level 触发器。 row-level 触发器注册包中的所有更改（包可以保存状态），而 statement-level 触发器跟进并根据行中发生的情况应用所有必要的修改--a statement-level触发器可以修改基础table.

以下是有关如何执行此操作的说明：Get rid of mutating table trigger errors with the compound trigger

在那篇文章中，Steven Feuerstein 不仅描述了问题的传统 package-based 解决方案，而且还提供了更现代的 compound-trigger-based 解决方案。

Answer 2

是的，要处理这种情况，您将需要以下触发器组合：

--First of all, You will need one package to hold the values:
CREATE OR REPLACE PACKAGE MY_VALUE_HOLDER AS
    POLICEMANID NUMBER;
    BOSS NUMBER;
END MY_VALUE_HOLDER;
/

-- Before each row trigger
CREATE OR REPLACE TRIGGER SWITCH_BOSS_ROW_TRG BEFORE
    DELETE ON POLICEMAN
    FOR EACH ROW
DECLARE
    BOSS   NUMBER;
BEGIN
    BOSS := :OLD.BOSSID;
    IF ( :OLD.POLICEMANID = :OLD.BOSSID ) THEN
        SELECT
            POLICEMANID
        INTO BOSS
        FROM
            (
                SELECT
                    *
                FROM
                    POLICEMAN
                ORDER BY
                    DBMS_RANDOM.VALUE
            )
        WHERE
            ROWNUM = 1;

    END IF;

    MY_VALUE_HOLDER.POLICEMANID := :OLD.POLICEMANID;
    MY_VALUE_HOLDER.BOSS := BOSS;
END SWITCH_BOSS_ROW_TRG;
/


--After statement trigger
CREATE OR REPLACE TRIGGER SWITCH_BOSS_ST_TRG AFTER
    DELETE ON POLICEMAN
BEGIN
    UPDATE POLICEMAN
    SET
        BOSSID = MY_VALUE_HOLDER.BOSS
    WHERE
        BOSSID = MY_VALUE_HOLDER.POLICEMANID;

END SWITCH_BOSS_ST_TRG;
/

第一个触发器在每行之前，第二个在语句触发器之后。

DB Fiddle demo

干杯！！

在触发器中使用更新

using update in trigger

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