Ok, as I understand, you want the first query
select sum(*) as x from xxx group by yyy
this will select differemnt sum for each yyy from table xxx.
select sum(*) from xxx will select the sum of all elements.
If you want a subquery, I don't know exactly how is it in access but in SQLServer you can do
select sum(t.x)
from
(select sum(*) as x from xxx group by yyy) as t
John Fill