create table #temp(f1 int, f2 varchar(10))

insert into #temp values(1, 'a')
insert into #temp values(2, 'b')
insert into #temp values(4, 'c')
insert into #temp values(5, 'd')
insert into #temp values(7, 'e')
insert into #temp values(9, 'f')

select * from #temp a left outer join #temp b on (a.f1+1) = b.f1 where b.f1 is null

select min(a.f1+1) from #temp a left outer join #temp b on (a.f1+1) = b.f1 where b.f1 is null

drop table #temp