oracle创建triger

create or replace trigger TRIGGER_T_O_MOULD_ID--trigger名称
  before insert on T_O_MOULD--表名
  for each row
begin
  if (:new.MOULD_ID is null) then --列
    select SN_SYSTEM.nextval into :new.MOULD_ID from dual; --列名
  end if;
end;