User Sessions | Month's Since Last Active Using SQL - postgresql

Question

UserID
CalMonth
ActiveFlag
Months_since_last_active
A
1/1/2021
1
0
A
2/1/2021
1
A
3/1/2021
2
A
4/1/2021
1
0
B
1/1/2021
1
0
B
2/1/2021
1
B
3/1/2021
1
0
Problem --> The first 3 colums are given. Generate the last one 'Months_since_last_active' by adding 1 until the use is active again
My Solution as below:
With active_sessions as (
Select
User_Id
, CalMonth
, active flag as current_flag
, LAG (ActiveFlag,1) over (partition by User_Id order by CalMonth) as previous_flag
)
Select User_Id, CalMonth, current_flag, sum(case when current_flag =1 then 0
when current_flag IS NULL then Months_since_last_active + 1
END
) as Months_since_last_active
from active_sessions
order by 1,2
I was asked the above question in an interview and told that my proposed solution would not work because:
When it comes to 3/1/2021 and beyond, the previous values of 'Months_since_last_active' are not in the table yet -- they are only in the code
If I wanted to use LAG function, then it'd take innumerable LAG functions to achieve what I was trying to achieve
I will appreciate if someone can comment on my solution.

Answer 1

Your solution has 3 major problems, 2 of them may be related to copy/past errors. The active_sessions CTE is missing the from clause, so there is no data source. Then the main portion uses the aggregate function SUM, however, the query has no group by which is required for the aggregate function. These are easily corrected. The other issue concerns the LAG function and your use of it.
First off in the CTE you alias the result as previous_flag, then in the main query you reference Months_since_last_active which does not exist yet. I think this is the source of the interviewer's first point.
The interviewer's second point also stems form the LAG function. As written it always looks back exactly 1 row, but from the current row yet it needs to look back 2 rows for (userid, calmonth) = ('A', 2021-03-01), and 3 rows for (A, 2021-04-01), etc. Basically you need to look back to to the last row with active_flag = 1. This leads directly to the it'd take innumerable LAG functions as you do not know how far beck you need to look. Suppose you had 30-40 or more inactive rows between active rows. You need a LAG(activeflag,n) ... for each possibility.
A solution. I dislike the problem statement it should not contain by adding 1 until the use is active again (is it yours or theirs). Either way this is an XY. If theirs they should be telling you what to solve, i.e. find number of months since last active. If yours you have created the problem for yourself. The problem statement should not say anything about how to solve the it. I will ignore that portion of the problem (And in a real interview I would/have ignored it, but be prepared to explain why).
What you have a a version of a Gaps And Islands (google it, you will find more that to think about). In this version lets consider each row with activeflag = 'Y' an as island, and anything else as a gap. Nor what you are looking for is the length of the gaps between islands. In the following the island_num CTE does 2 things. It assigns a sequence number to each row for a (userid, calmonth) and generates a boolean for each island. The `gap_points' then joins the results with itself, selecting the assigned for the max island whose calmonth is less than the current rows calmonth. In the main part the Months_since_last_active is assigned 0 if the current row is an island, and the difference between the generated row numbers if it is a gap. (see demo)
with island_num (userid, cal_month, active_flag, is_island, row_num) as
( select am.*
, case when am.activeflag = 1 then true else false end is_island
, row_number() over (partition by am.userid order by am.calmonth) rn
from active_month am
) -- select * from island_num
, gap_points(userid, cal_month, active_flag, is_island, row_num, island_row) as
( select *
from island_num i1
join lateral
(select max(row_num)
from island_num i2
where i1.userid = i2.userid
and i2.cal_month < i1.cal_month
and i2.is_island
) s0
on true
) --select * from gap_points;
select userid "User Id"
, cal_month "Cal Month"
, active_flag "Active Flag"
, case when is_island then 0
else row_num - island_row
end "Months_since_last_active"
from gap_points;