Cumulative distinct count filtered by last value - T-SQL - sql

Question

I am trying to come up with exactly the same answer as here:
Cumulative distinct count filtered by last value - DAX
but in SQL Server. For convenience I am copying the whole problem description.
I have a dataset:
month name flag
1 abc TRUE
2 xyz TRUE
3 abc TRUE
4 xyz TRUE
5 abc FALSE
6 abc TRUE
I want to calculate month-cumulative distinct count of 'name' filtered by last 'flag' value (TRUE). I.e. I want to have a result:
month count
1 1
2 2
3 2
4 2
5 1
6 2
In months 5 and 6 'abc' should be excluded because the flag switched to 'FALSE' in month 5.
I am thinking about using "over" clause with "partition by" but I don't have any experience here so it's a struggle for me.
UPDATE
I have updated the last row in exemplary source data.
was:
6 abc FALSE
is:
6 abc TRUE
And the last row in output data.
Was:
6 1
is:
6 2
It might have not been obivous from the description that it should work this way and the proposed answer does not solve this problem.
UPDATE 2
I have managed to create a query that gives the result but it's ugly and I think could be shrinked by using over clause. Can you help me with that?
select t5.month_current, count(*) as count from
(select t3.month month_current, t4.month months_until_current, t3.name, t4.flag from
(select name ,month from
(select distinct name
from Source_data) t1
,(select distinct month
from Source_data) t2) t3
left join
Source_data t4
on t3.name = t4.name and t3.month >= t4.month) t5
inner join
(select t3.month month_current, max(t4.month) real_max_month_until_current, t3.name from
(select name ,month from
(select distinct name
from Source_data) t1
,(select distinct month
from Source_data) t2) t3
left join
Source_data t4
on t3.name = t4.name and t3.month >= t4.month
group by
t3.month, t3.name) t6
on t5.month_current = t6.month_current
and t5.months_until_current = t6.real_max_month_until_current
and t5.name = t6.name
where t5.flag = 'TRUE'
group by t5.month_current

Answer 1

You can do a cumulative distinct count as:
select t.*,
sum(case when seqnum = 1 then 1 else 0 end) over (order by month) as cnt
from (select t.*,
row_number() over (partition by name order by month) as seqnum
from t
) t;
I don't understand the logic for incorporating the flag.
You can replicate the results in the question by incorporating the flag:
select t.*,
sum(case when seqnum = 1 and flag = 'true' then 1
when seqnum = 1 and flag = 'false' then -1
else 0
end) over (order by month) as cnt
from (select t.*,
row_number() over (partition by name, flag order by month) as seqnum
from t
) t;