create table #yourTable (col1 char(1), col2 int)
insert #yourTable 
select 'A', 10 union all
select 'A', 20;

with AllVals as (
	select	a
		,n
	from	(
		select	char(v.number) a
		from	master..spt_values v
		where	type = 'p'
		and	number between 65 and 90
		) alph
	cross join (select 10 n union all select 20 union all select 30 union all select 40) num
	)
select AllVals.a, AllVals.n
from	AllVals
left join #yourTable yt on yt.col1 = AllVals.a and yt.col2 = AllVals.n
where	yt.col1 is null
and AllVals.a = 'A' --Just the "A"s for this example

drop table #yourTable

output:
a    n
---- -----------
A    30
A    40

create table #yourTable (col1 char(1), col2 int)
insert #yourTable 
select 'A', 10 union all
select 'A', 20;


select AllVals.a, AllVals.n
from	(
	select	a
		,n
	from	(
		select	char(v.number) a
		from	master..spt_values v
		where	type = 'p'
		and	number between 65 and 90
		) alph
	cross join (select 10 n union all select 20 union all select 30 union all select 40) num
	) allVals
left join #yourTable yt on yt.col1 = AllVals.a and yt.col2 = AllVals.n
where	yt.col1 is null
and AllVals.a = 'A' --Just the "A"s for this example

drop table #yourTable

output:
a    n           
---- ----------- 
A    30
A    40