select a , b , newcol = b+c from tabletemp where flag = '是' 
union  all 
select a , b , newcol = b+f from tabletemp where flag = '否'
order by id

------解决方案--------------------
select   a,b,b+c   from   tabletemp where flag='是'  
union
select   a,b,b+c   from   tabletemp flag='否'  
order by id

------解决方案--------------------
SQL code
--一共三个,你看哪个合适?
select a , b , newcol from
(
  select id , a , b , newcol = b+c from tabletemp where flag = '是' 
  union  all 
  select id , a , b , newcol = b+f from tabletemp where flag = '否'
) t
order by id

------解决方案--------------------
select a , b , newcol = b+ case when flag = '是' then c else f end
from tabletemp

------解决方案--------------------
select a , b , newcol = b+ case when flag = '是' then c else f end
from tabletemp
order by id