Average of last 3 months (SQL vertica) - sql

Question

I need to find the average days past due for the last 3 months for each client. Not as a rolling/moving average, but one time number, always calculating the last 3 months, no matter if the data changes.
For example now the last data I have is from Sept 2022, so I need the average of Sept 2022, August 2022 and July 2022. But if the data changes and now I have October 2022, then I would need average of Oct, Sept, August and so on.
I tried this but it calculates wrong
CREATE TABLE AVERAGE_dpd
AS (
SELECT "SUM_WEIGHTED_AVG_PERMONTH"."NAME",
AVG("SUM_WEIGHTED_AVG_PERMONTH"."SUM")
OVER (PARTITION BY "SUM_WEIGHTED_AVG_PERMONTH"."NAME"
order by MONTH ("SUM_WEIGHTED_AVG_PERMONTH"."LAST DAY OF MONTH_NETDUEDATE") desc
rows between 2 preceding and CURRENT ROW)
as AVG3Months
FROM "SUM_WEIGHTED_AVG_PERMONTH");
Thank you so much for your help!

Answer 1

I assume you are expecting this result:
client_name | avg_last_3mth
-------------+--------------
Client 01 | 16.76
Client 02 | 5.75
Client 03 | -13.95
So I assume you have something like this as input data (and this is how we usually like the data to accompany the question):
month_begin | client_name | dpd
------------+-------------+----
2022-01-05 | Client 01 | 12
2022-01-06 | Client 01 | 14
2022-01-07 | Client 01 | 18
2022-01-08 | Client 01 | 17
2022-01-05 | Client 02 | 12
2022-01-06 | Client 02 | 14
2022-01-07 | Client 02 | 18
2022-01-08 | Client 02 | 17
2022-01-05 | Client 03 | 12
2022-01-06 | Client 03 | 14
2022-01-07 | Client 03 | 18
2022-01-08 | Client 03 | 17
With this input data, you probably want the rows with month_begin of the first of the current month (TRUNC(CURRENT_DATE,'MONTH')), plus the two previous months. And this is what I do, then I obviously group by the client name:
WITH
-- input data I made up, don't use in query ..
dpd(month_begin,client_name,dpd) AS (
SELECT DATE '2022-05-01','Client 01',12
UNION ALL SELECT DATE '2022-06-01','Client 01',14
UNION ALL SELECT DATE '2022-07-01','Client 01',18
UNION ALL SELECT DATE '2022-08-01','Client 01',17
UNION ALL SELECT DATE '2022-05-01','Client 02', 2
UNION ALL SELECT DATE '2022-06-01','Client 02', 4
UNION ALL SELECT DATE '2022-07-01','Client 02', 8
UNION ALL SELECT DATE '2022-08-01','Client 02', 7
UNION ALL SELECT DATE '2022-05-01','Client 03',22
UNION ALL SELECT DATE '2022-06-01','Client 03',24
UNION ALL SELECT DATE '2022-07-01','Client 03',28
UNION ALL SELECT DATE '2022-08-01','Client 03',27
)
-- real query starts here ..
SELECT
client_name
, AVG(dpd)::NUMERIC(5,2) AS avg_last_3mth
FROM dpd
WHERE month_begin >= TRUNC(CURRENT_DATE,'MONTH') - '2 MONTHS'::INTERVAL YEAR TO MONTH
GROUP BY
client_name;
-- out client_name | avg_last_3mth
-- out -------------+---------------
-- out Client 02 | 6.33
-- out Client 01 | 16.33
-- out Client 03 | 26.33