with billing(ID, contract_id, bill_sum, bill_date, bill_type)
as (
select 25643, 322456,  18800, date '2020-06-02', 'INCOMING' from dual union all
select 25643, 322458,  15645, date '2020-11-22',  'SMS' from dual union all
select 452632,  322459,  8403,  date '2019-02-23',  'SMS' from dual union all
select 4526,  322459,  8403,  date '2020-01-23',  'SMS' from dual union all
select 9874, 322460,  24535, date '2020-01-10',  'SMS' from dual union all
select 98744, 322460,  24535, date '2018-04-07',  'SMS' from dual union all
select 3032976, 322461,  14000, date '2012-09-17',  'Roaming' from dual union all
select 3032976, 322459,  14000, date '2020-03-17',  'Roaming' from dual union all
select 304598,  322462,  17326, date '2020-06-12',  'SMS' from dual union all
select 6439832, 3537962, 6784,  date '2012-08-17',  'DUMMY' from dual
)

select * from billing b
where b.bill_date between date '2020-01-01' and date '2020-01-31'
-- and not exists (select 1 from billing b2 where b2.contract_id = b.contract_id and b2.bill_date between date '2020-03-01' and date '2020-03-31')

select count(distinct contract_id) from billing b2 where b2.bill_date between date '2019-01-01' and date '2019-12-31'
union all
select count(distinct contract_id) from billing b

select (select count(distinct contract_id) from billing where bill_date between date '2019-01-01' and date '2019-12-31') v1,
       count(distinct contract_id) v2
  from billing b

select contract_id, smski
  from (
select contract_id, count(1) smski
  from billing b
where b.bill_date between date '2020-01-01' and date '2020-12-31'
and b.bill_type = 'SMS'
group by contract_id
      ) order by smski desc

and b.bill_type != 'DUMMY'