sql count new id that did not exists before for each month - sql

Question

I have the follow set of data
enter image description here
how can I write the sql to gives the result on right side?
that is the counting of unique id that did appeared previously for each month.

Answer 1

After long time of reading and reading his question, Ssiu wanted to ask the following:
So here is the test data in MS SQL: at that time he didn't clarify on postgresql
create table tmp1 (
ddate datetime
, iid int
)
insert into tmp1 values
('2017-11-01',1)
,('2017-11-02',2)
,('2017-11-03',3)
,('2017-11-04',4)
,('2017-11-05',5)
,('2017-11-06',5)
,('2017-11-07',5)
,('2017-12-01',1)
,('2017-12-02',2)
,('2017-12-03',3)
,('2017-12-04',6)
,('2017-12-05',7)
,('2018-01-01',1)
,('2018-01-02',2)
,('2018-01-03',3)
,('2018-01-04',4)
,('2018-01-05',8)
Disclaimer: The following is not the best approach for this problem. It is not applicable for more months, however it can give Ssiu a clue.
with cte(mmonth, iid) as (
select distinct convert(varchar(7), ddate, 120) mmonth
, iid
from tmp1
)
, cte_201711 as (
select * from cte where mmonth = '2017-11'
)
, cte_201712 as (
select * from cte where mmonth = '2017-12'
)
, cte_201801 as (
select * from cte where mmonth = '2018-01'
)
, cte_cnt201712 as(
select cte_201711.mmonth as mm201711
, cte_201711.iid as id201711
, cte_201712.mmonth as mm201712
, cte_201712.iid as id201712
from cte_201711
full outer join cte_201712
on cte_201712.iid = cte_201711.iid
)
, cte_cnt201801 as (
select cte_201711.mmonth as mm201711
, cte_201711.iid as id201711
, cte_201712.mmonth as mm201712
, cte_201712.iid as id201712
, cte_201801.mmonth as mm201801
, cte_201801.iid as id201801
from cte_201711
full outer join cte_201712
on cte_201712.iid = cte_201711.iid
full outer join cte_201801
on cte_201801.iid = cte_201712.iid
or cte_201801.iid = cte_201711.iid
)
--select * from cte_cnt201801 order by isnull(mm201711,'z'), isnull(mm201712,'z')
select '2017-12' mmonth, count(*) Ssiu
from cte_cnt201712
where mm201711 is null
union all
select '2018-01' mmonth, count(*) Ssiu
from cte_cnt201801
where mm201711 is null
and mm201712 is null
Note the data for the cte_cnt201801 CTE:
select * from cte_cnt201801 order by isnull(mm201711,'z'), isnull(mm201712,'z')
So the result for the above query is: