using update in trigger

I have trigger on delete I want to delete policeman but when I delete him all other policeman that he is their boss will have null in their boss field so I used this code

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;

I got error

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'

any ideas?

UPDATE: I used compund trigger its works but not as I wanted. I wanted to set the boss of the deleted policeman as the boss for the ones he was boss of. the problem is when deleting I cant now which policemans have the deleted policem as boss. I can find them because they have null in the field after the delete but they may belong to other deleted policeman.

this is the trigger I made:

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

This is a very common problem when getting started with triggers. Oracle does not let your row-level trigger access its base table (POLICEMAN) other than through the :NEW and :OLD pseudo-records. The idea is that the table is in a state of flux, so it cannot be queried or modified by the trigger..

A typical way of handling this is to create two triggers: a row-level trigger followed by a statement-level trigger. The row-level trigger registers all of the changes in a package (packages can hold state), while the statement-level trigger follows up and applies all of the necessary modifications based on what happened in the rows–a statement-level trigger can modify the base table.

Here is a description on how to do this: Get rid of mutating table trigger errors with the compound trigger

In that article, Steven Feuerstein describes not only the traditional package-based solution to the problem, but he also provides a more modern compound-trigger-based solution.