How can I partition by group that falls within a time range?

I have the following table showing when customers bought a certain product. The data I have is CustomerID, Amount, Dat. I am trying to create the column ProductsIn30Days, which represents how many products a customer bought in the range Dat-30 days inclusive the current day.

For example, ProductsIn30Days for CustomerID 1 on Dat 25.3.2020 is 7, since the customer bought 2 products on 25.3.2020 and 5 more products on 24.3.2020, which falls within 30 days before 25.3.2020.

CustomerID	Amount	Dat	ProductsIn30Days
1	1	23.3.2018	1
1	2	24.3.2020	2
1	3	24.3.2020	5
1	2	25.3.2020	7
1	2	24.5.2020	2
1	1	15.6.2020	3
2	7	24.3.2017	7
2	2	24.3.2020	2

I tried something like this with no success, since the partition only works on a single date rather than on a range like I would need:

select CustomerID, Amount, Dat,
sum(Amount) over (partition by CustomerID, Dat-30)
from table

Thank you for help.

Answer

You can use an analytic SUM function with a range window:

SELECT t.*,
       SUM(Amount) OVER (
         PARTITION BY CustomerID
         ORDER BY Dat
         RANGE BETWEEN INTERVAL '30' DAY PRECEDING AND CURRENT ROW
       ) AS ProductsIn30Days
FROM   table_name t;

Which, for the sample data:

CREATE TABLE table_name (CustomerID, Amount, Dat) AS
SELECT 1, 1, DATE '2018-03-23' FROM DUAL UNION ALL
SELECT 1, 2, DATE '2020-03-24' FROM DUAL UNION ALL
SELECT 1, 3, DATE '2020-03-24' FROM DUAL UNION ALL
SELECT 1, 2, DATE '2020-03-25' FROM DUAL UNION ALL
SELECT 1, 2, DATE '2020-05-24' FROM DUAL UNION ALL
SELECT 1, 1, DATE '2020-06-15' FROM DUAL UNION ALL
SELECT 2, 7, DATE '2017-03-24' FROM DUAL UNION ALL
SELECT 2, 2, DATE '2020-03-24' FROM DUAL;

Outputs:

CUSTOMERID	AMOUNT	DAT	PRODUCTSIN30DAYS
1	1	2018-03-23 00:00:00	1
1	2	2020-03-24 00:00:00	5
1	3	2020-03-24 00:00:00	5
1	2	2020-03-25 00:00:00	7
1	2	2020-05-24 00:00:00	2
1	1	2020-06-15 00:00:00	3
2	7	2017-03-24 00:00:00	7
2	2	2020-03-24 00:00:00	2

Note: If you have values on the same date then they will be tied in the order and always aggregated together (i.e. rows 2 & 3). If you want them to be aggregated separately then you need to order by something else to break the ties but that would not work with a RANGE window.

db<>fiddle here