update a 
set a.timeA=b.timeA
from ta as a,tb as b where a.关联条件=b.关联条件
and not exists(select 1 from tb where 关联条件=b.关联条件 and time1>b.time1)

------解决方案--------------------
update A set timeA = case
   when time1 is null then time2
   when time2 is null then them1
   when time1 < time2 then time2
   else time1 end
from A
join B on A.id=B.id -- 假设A,B表以ID关联


------解决方案--------------------
SQL code

update A set timeA = case
  when time1 is null then time2
  when time2 is null then them1
  when time1 < time2 then time2
  else time1 end
from b
where  A.关联关系=B.关联关系

------解决方案--------------------
SQL code

update A set timeA = case
  when time1 is null then time2
  when time2 is null then them1
  when time1 < time2 then time2
  else time1 end
from b
where  A.关联关系=B.关联关系