Count number of members each month - sql

Question

I have a table like so
--Member--
ID char(10),
Name (nvarchar(50),
joiningDay date,
exitDay date
How can I count total number of member each month with a selected period of time with member that has exit will not be counted in that month
and the result should be like:
Start from month 1 to month 10
Month totalOfMember
1 5
2 6
3 9
... ...
10 35

Answer 1

If you want to select number of members joined in a month
select month(joiningday)[Month],sum(case when month(joiningday)=month(exitday) then 0 else 1 end)TotalOfMember
from Member where joiningDay>='1 jan 2020' and joiningDay<'1 jan 2021'
group by month(joiningday)
order by [Month]
But I would suggest to use Format(datevalue,'yyyy MM') to count member month and year wise instead of only month. 01 can be repeated in a date range if there are two January but 2020 01 and 2021 01 will be uniquely identified.
select Format(joiningday,'yyyy MM') [Month],sum(case when Format(joiningday,'yyyy MM') = Format(exitday,'yyyy MM') then 0 else 1 end)TotalOfMember
from Member where joiningDay>='1 jan 2020' and joiningDay<'1 jan 2021'
group by Format(joiningday,'yyyy MM')
order by [Month]
Or if you want to have month wise cumulative count for members :
select Format(joiningday,'yyyy MM') [Month],sum(case when Format(joiningday,'yyyy MM') = Format(exitday,'yyyy MM') then 0 else 1 end)over(order by Format(joiningday,'yyyy MM')) TotalOfMember
from Member where joiningDay>='1 jan 2020' and joiningDay<'1 jan 2021'
group by Format(joiningday,'yyyy MM')
order by [Month]

Answer 2

Join the table to itself to eliminate same-month exiters:
select
year(a.joinjngday) Year,
month(a.joiningday) Month,
count(*) total
from Member a
join Member b on a.ID = b.ID
and month(b.exitday) = month(a.joinjngday)
and year(b.exitday) = year(a.joiningday)
where b.ID is null -- exclude same month exits
group by month(a.joiningday)
order by year(a.joinjngday), month(a.joiningday)

Answer 3

The most efficient way to do this is to use a cumulative sum. Basically, you want a + count and - count for each month based on the number of members who join and leave. I would approach this as:
with changes as (
select dateformparts(year(dte), month(dte), 1) as yyyymm,
sum(inc) as month_changes
from members m cross apply
(values (joiningday, 1),
(exitday, -1)
) v(dte, inc)
)
select yyyymm, sum(month_changes)
from changes
group by yyyymm
order by yyyymm;
If you want this for a particular period of time, then filter the above. Also, this will not contain months that have no changes.
If you wanted this for just a handful of months -- even those with no changes -- then you might find that explicit counting is simpler:
select d.dte,
(select count(*)
from members m
where m.joiningday <= eomonth(d.dte) and
m.exitday > eomonth(d.dte)
) as members_in_month
from (values (convert(date, '2021-01-01')),
(convert(date, '2021-02-01')),
(convert(date, '2021-03-01'))
) d(dte);