Unique cumulative customers by each day - sql

Question

Task: Get the total number of unique cumulative customers by each decline reason and by each day.
Input data sample:
+---------+--------------+------------+------+
| Cust_Id | Decline_Dt | Reason | Days |
+---------+--------------+------------+------+
| A | 08-09-2020 | Reason_1 | 0 |
| A | 08-09-2020 | Reason_1 | 1 |
| A | 08-09-2020 | Reason_1 | 2 |
| A | 08-09-2020 | Reason_1 | 4 |
| B | 08-09-2020 | Reason_1 | 0 |
| B | 08-09-2020 | Reason_1 | 2 |
| B | 08-09-2020 | Reason_1 | 3 |
| C | 08-09-2020 | Reason_1 | 1 |
+---------+--------------+------------+------+
1) Decline_dt - The date on which the payment was declined. (Ignore it for this task)
2) Days - Indicates the # of days after the payment decline happened, the customer interacted with IVR channel.
3) Reason - Indicates the payment decline reason
--Expected Output:
+---------------+-----------+---------------+----------------------------+
| Reason | Days | Unique_mtns | total_cumulative_customers |
+---------------+-----------+---------------+----------------------------+
| Reason_1 | 0 | 2 | 2 |
| Reason_1 | 1 | 2 | 3 |
| Reason_1 | 2 | 2 | 3 |
| Reason_1 | 3 | 1 | 3 |
| Reason_1 | 4 | 1 | 3 |
+------------------------------------------------------------------------+
My Hive query:
select a.Reason
, a.days
-- , count(distinct a.cust_id) as unique_mtns
, count(distinct a.cust_id) over (partition by Reason
order by a.days rows between unbounded preceding and current row)
as total_cumulative_customers
from table as a
group by a.reason
, a.days
Output (Incorrect):
+---------------+-----------+----------------------------+
| Reason | Days | total_cumulative_customers |
+---------------+-----------+----------------------------+
| Reason_1 | 0 | 2 |
| Reason_1 | 1 | 2 |
| Reason_1 | 2 | 2 |
| Reason_1 | 3 | 1 |
| Reason_1 | 4 | 1 |
+--------------------------------------------------------+
Ideally, I would expect the window function to be executed without group by.
However, I get an error without group by. When I use group by, I don't get the cumulative customers.

Answer 1

If I follow you correctly, you can use a subquery to compute the first day per customer/reason tuple, and then do conditional aggregation:
select reason, days,
count(distinct cust_id) as unique_mtns,
sum(sum(case when days = min_days then 1 else 0 end))
over(partition by reason order by days) as total_cumulative_customers
from (
select reason, cust_id,
min(days) over(partition by reason, cust_id) as min_days
from mytable
) t
group by reason, days

Answer 2

I would recommend using row_number() to enumerate the rows or a given customer and reason. Your code uses count(distinct) on the user id, suggesting that you might have duplicates on a given day.
This would be:
select reason, days, count(distinct cust_id) as unique_mtns,
sum(sum(case when seqnum = 1 then 1 else 0 end)) over (partition by reason order by days) as total_cumulative_customers
from (select t.*,
row_number() over (partition by reason, cust_id order by days) as seqnum
from t
) t
group by reason, days
order by reason, days;