with countf as (
    select nationid, count(*) as c from customer
    group by nationid
),
maxf as ( select max(nationid) from customer )

select c.customerid, c.nationid from customer c, countf cf, maxf m
where c.nationid = cf.nationid
and cf.c = m

cf.c = ( select max(nationid) from customer )

cf.c in maxf