select
x.Domain,
x.No_of_users
from
(select
d.Domain,
count(d.Domain) as No_of_users
from
Domain d
group by
d.Domain) x
where
x.No_of_users =
(select
max(x2.No_of_users)
from
(select
d2.Domain,
count(d2.Domain) as No_of_users
from
Domain d2
group by
d2.Domain) x2)