create table #flat(numbalcon int null, numlodgia int null)
insert into #flat values( 2 , null)
insert into #flat values(null,  3 )
insert into #flat values( 2 ,  3 )

select   --вариант 1: результат деления
 
case 
    when not(numbalcon is null) and not(numlodgia is null)
	then convert(float, numbalcon)/convert(float, numlodgia)
    when not(numbalcon is null) and numlodgia is null
	then numbalcon
    when numbalcon is null and not(numlodgia is null)
	then numlodgia
    end
from #flat
 /*  2.0
    3.0
    0.66666666666663*/ 

select   --вариант 2: строковое представление
 
case 
    when not(numbalcon is null) and not(numlodgia is null)
	then convert(varchar( 4 ), numbalcon) + '/'+ convert(varchar( 4 ), numlodgia)
    when not(numbalcon is null) and numlodgia is null
	then convert(varchar( 4 ),numbalcon)
    when numbalcon is null and not(numlodgia is null)
	then convert(varchar( 4 ),numlodgia)
    end
from #flat
 /*  2
    3
    2/3 */ 

drop table #flat