How to get New, Existing and Inactive users from table - sql

Question

For example, Below is input table which has Month & User
Output Required:
NewUsers are new in that month. ExistingUsers are users in that month which have some data in previous month as well. Inactive users are users active in previous month but not in current month
Is it possible?

Answer 1

You can use windowed function to achieve that:
New User is very easy COUNT rows that have rn = 1
Existing Users: easy too, COUNT rows that have rn > 1
Inactive Users: bit complicated (get sum of new + existing and substract (new + existing) from row before.
Code:
WITH cte AS
(
SELECT *
,rn = ROW_NUMBER() OVER (PARTITION BY UserKey ORDER BY MonthId)
FROM #tab t1
), cte2 AS(
SELECT
MonthId,
[New_User] = COUNT(CASE WHEN rn = 1 THEN 1 END),
[Existing_User] = COUNT(CASE WHEN rn > 1 THEN 1 END),
[s] = COUNT(rn)
FROM cte
GROUP BY MonthId
)
SELECT
MonthId,
[New_User],
[Existing_User],
[Inactive_User] = CASE WHEN [s] - LAG(s, 1) OVER(ORDER BY MonthId) < 0
THEN ABS([s] - LAG(s, 1) OVER(ORDER BY MonthId))
ELSE 0
END
FROM cte2
ORDER BY MonthId;
LiveDemo
Output:
╔═════════╦═══════════╦════════════════╦════════════════╗
║ MonthID ║ New_Users ║ Existing_Users ║ Inactive_Users ║
╠═════════╬═══════════╬════════════════╬════════════════╣
║ 201411 ║ 1 ║ 0 ║ 0 ║
║ 201412 ║ 1 ║ 1 ║ 0 ║
║ 201501 ║ 1 ║ 2 ║ 0 ║
║ 201502 ║ 0 ║ 2 ║ 1 ║
╚═════════╩═══════════╩════════════════╩════════════════╝
Warning:
I've assumed that data per each MonthId is UNIQUE if not add one more CTE step to remove duplicates first.