如果更新这样的关系表？

create or replace trigger
  before update or delete or insert on A
  for each row
begin
  --当A更新时触发
  if updating then
    update B
       set b2 = ....自定义,例如：:new.a2
     where B.id = :new.id;
  end if;

  --当A删除时触发
  if deleting then
    delete from B
     where B.id = :old.id;
  end if;  --当A插入时触发
  if inserting then
    insert into B
      (id,b2) values (:new.id,a2);
  end if;
end;