我试图获取一个触发器，用当前时间戳更新一行的值，并且该触发器插入的表中正在使用相同的时间戳。触发器与set new.last\u update='some constant'一起工作，但添加变量或select all时，行中的值为null。

drop trigger if exists event_update_history;
delimiter //
create trigger event_update_history before update on Events 
    for each row
    begin
        declare history_timestamp timestamp;
        set @history_timestamp = timestamp(unix_timestamp());
        if (new.current_state <> old.current_state) then
            insert into Event_History (evt_id, evt_state, time_of_change, userid) values (new.evt_id, new.current_state, history_timestamp, new.last_updated_by);
        end if;
        set new.last_update = history_timestamp; ## THIS IS THE PART THAT IS NOT WORKING but works if you send a constant
    end; //
delimiter ;