Finding Active Clients By Date - sql

Question

I'm having trouble writing a recursive function that would count the number of active clients on any given day.
Say I have a table like this:
Client
Start Date
End Date
1
1-Jan-22
2
1-Jan-22
3-Jan-22
3
3-Jan-22
4
4-Jan-22
5-Jan-22
5
4-Jan-22
6-Jan-22
6
7-Jan-22
9-Jan-22
I want to return a table that would look like this:
Date
NumActive
1-Jan-22
2
2-Jan-22
2
3-Jan-22
3
4-Jan-22
4
5-Jan-22
4
6-Jan-22
3
7-Jan-22
3
8-Jan-22
3
9-Jan-22
4
Is there a way to do this? Ideally, I'd have a fixed start date and go to today's date.
Some pieces I have tried:
Creating a recursive date table
Truncated to Feb 1, 2022 for simplicity:
WITH DateDiffs AS (
SELECT DATEDIFF(DAY, '2022-02-02', GETDATE()) AS NumDays
)
, Numbers(Numbers) AS (
SELECT MAX(NumDays) FROM DateDiffs
UNION ALL
SELECT Numbers-1 FROM Numbers WHERE Numbers > 0
)
, Dates AS (
SELECT
Numbers
, DATEADD(DAY, -Numbers, CAST(GETDATE() -1 AS DATE)) AS [Date]
FROM Numbers
)
I would like to be able to loop over the dates in that table, such as by modifying the query below for each date, such as by #loopdate. Then UNION ALL it to a larger final query.
I'm now stuck as to how I can run the query to count the number of active users:
SELECT
COUNT(Client)
FROM clients
WHERE [Start Date] >= #loopdate AND ([End Date] <= #loopdate OR [End Date] IS NULL)
Thank you!

Answer 1

You don't need anything recursive in this particular case, you need as a minimum a list of dates in the range you want to report on, ideally a permanent calendar table.
for purposes of demonstration you can create something on the fly, and use it like so, with the list of dates something you outer join to:
with dates as (
select top(9)
Convert(date,DateAdd(day, -1 + Row_Number() over(order by (select null)), '20220101')) dt
from master.dbo.spt_values
)
select d.dt [Date], c.NumActive
from dates d
outer apply (
select Count(*) NumActive
from t
where d.dt >= t.StartDate and (d.dt <= t.EndDate or t.EndDate is null)
)c
See this Demo Fiddle