oracle中根据时间获取最新的一条数据

1、select kd.CREATEUSERID as userid,kd.LOCATION,kd.createtime as location from KT_DEVICESTRACK kd where rownum=1 order by kd.createtime

2、SELECT * FROM (
SELECT *,ROWNUM rn FROM t ORDER BY date_col DESC
) WHERE rn = 1

3、update cg_berthtrace cg set cg.chagedtime=sysdate,cg.parkduration=(cg.chagedtime-cg.createtime)*24*60,cg.berthstatus='11'
 where cg.berthno in(select po.bowei from pda_order po where po.starttime=(select max(starttime) from pda_order))