select count(*)from(select tel_num, manufacturer from abcd
where manufacturer='big_company'
and tel_num is not null)pqr

select cast (count (*)as bigint) from (select a.tel_num, b.postpaid from
abcd as a inner join wxyz as b on a.tel_num=b.tel_num
where a.manufacturer='big_company'
and b.postpaid=1) xyz

select cast (count (*)as bigint)
from abcd a
where exists (select 1 from wxyz b
  where a.tel_num=b.tel_num and b.postpaid=1)
and a.manufacturer='big_company'