Oracle 30秒后的系统时间

　　select sysdate,sysdate+1/24/60/60*30 from dual;　　select sysdate,sysdate+30/(24*60*60) from dual;　　select sysdate,sysdate+(0.5/24/60) from dual;　　select sysdate,sysdate+(interval '30' second) from dual;