How to Add one row to another - sql

Question

I have a set of data which is shown in picture:
I've tried adding January to February using the lead function which was correct shown in the picture below again:
And I thought it will add and add to the next row like a for loop but it did not.
I am asking for help on how to add the data again and again like a for loop would do. I've pasted the code I used below.
;WITH MonthActiveUsers AS (
SELECT MonthLogin, COUNT(MonthLogin) AS ActiveUsers
FROM #TEMPDistinctActiveUsersPerMonth DAUPM
GROUP BY MonthLogin
)
SELECT MAU.MonthLogin, (LEAD(MAU.ActiveUsers,0) OVER (ORDER BY MAU.MonthLogin ASC) + LEAD(MAU.ActiveUsers,1) OVER (ORDER BY MAU.MonthLogin ASC)) AS [Count of Active Users]
FROM
( SELECT CASE
WHEN MonthLogin = 'January'
THEN '2020-01-01'
WHEN MonthLogin = 'February'
THEN '2020-02-01'
WHEN MonthLogin = 'March'
THEN '2020-03-01'
WHEN MonthLogin = 'April'
THEN '2020-04-01'
WHEN MonthLogin = 'May'
THEN '2020-05-01'
WHEN MonthLogin = 'June'
THEN '2020-06-01'
ELSE NULL
END AS [MonthLogin]
,ActiveUsers
FROM MonthActiveUsers MAU
) MAU
The desired result would be:
January = 3313
February = 3349
March = 3398
April = 3421
May = 3437
June = 3444

Answer 1

I think that you want a window sum:
WITH ...
SELECT
MAU.MonthLogin,
SUM(MAU.ActiveUsers) OVER (ORDER BY MAU.MonthLogin) [Count of Active Users]
FROM ...
Note that the query can be simplified a little by using the short-circuit form of CASE, and mixing aggregation and window functions:
SELECT
MonthLogin,
COUNT(*) ActiveUsers,
SUM(COUNT(*)) OVER(ORDER BY MonthLogin) [Count of Active Users]
FROM (
SELECT
CASE MonthLogin
WHEN'January' THEN '2020-01-01'
WHEN'February' THEN '2020-02-01'
WHEN'March' THEN '2020-03-01'
WHEN'April' THEN '2020-04-01'
WHEN'May' THEN '2020-05-01'
WHEN'June' THEN '2020-06-01'
END AS MonthLogin
FROM #TEMPDistinctActiveUsersPerMonth
) t
GROUP BY MonthLogin
ORDER BY MonthLogin