--試下
select c.conference_id,t1.time1,t2.time2
from conference c
left join (select max(time1) time1 from time1 group by conference_id) t1 on c.conference_id=t1.conference_id
left join (select max(time2) time2 from time2 group by conference_id) t2 on c.conference_id=t2.conference_id

------解决方案--------------------

SQL code

--1、union 本身就能剔重
SELECT CONFERENCE_NAME,
       (SELECT MAX(TIME1)
          FROM TIME1 T1
         WHERE T1.CONFERENCE_ID = C.CONFERENCE_ID
        UNION
        SELECT MAX(TIME2)
          FROM TIME2 T2
         WHERE T2.CONFERENCE_ID = C.CONFERENCE_ID) MAX_TIME
  FROM CONFERENCE C

--2、用分析函数来做
SELECT CONFERENCE_NAME,
       GREATEST(MAX(T1.TIME1) OVER(), MAX(T2.TIME2) OVER()) MAX_TIME
  FROM CONFERENCE C, TIME1 T1, TIME2 T2
 WHERE T1.CONFERENCE_ID = C.CONFERENCE_ID
   AND T2.CONFERENCE_ID = C.CONFERENCE_ID;