Split value from a total row to multiple other rows until the sum reaches the value of the total row

Question

DB-Fiddle

CREATE TABLE inbound (
    id SERIAL PRIMARY KEY,
    campaign VARCHAR,
    expected_inbound_date DATE,
    expected_inbound_quantity DECIMAL,
    received_inbound_quantity DECIMAL
);

INSERT INTO inbound
(campaign, expected_inbound_date, expected_inbound_quantity, received_inbound_quantity)
VALUES 
('C001', '2022-05-03', '500', '0'),
('C001', '2022-05-03', '800', '0'),
('C001', '2022-05-03', '400', '0'),
('C001', '2022-05-03', '200', '0'),
('C001', NULL, '0', '700'),

('C002', '2022-08-20', '3000', '0'),
('C002', '2022-08-20', '5000', '0'),
('C002', '2022-08-20', '2800', '0'),
('C002', NULL, '0', '4000');

Expected Result

campaign |  expected_inbound_date |  expected_inbound_quantity  |  split_received_inbound_quantity
---------|------------------------|-----------------------------|----------------------------------
  C001   |        2022-05-03      |             200             |          200
  C001   |        2022-05-03      |             400             |          400
  C001   |        2022-05-03      |             500             |          100
  C001   |        2022-05-03      |             800             |            0
  C001   |                        |                             |          700
---------|------------------------|-----------------------------|----------------------------------
  C002   |       2022-08-20       |           3.800             |         3.800
  C002   |       2022-08-20       |           5.000             |           200
  C002   |       2022-08-20       |           2.800             |             0
  C002   |                        |                             |         4.000

---

I want to split the received_inbound_quantity to each row of the expected_inbound_quantity until the total of the received_inbound_quantity is reached.

SELECT 
i.campaign AS campaign,
i.expected_inbound_date AS expected_inbound_date,
i.expected_inbound_quantity AS expected_inbound_quantity,
i.received_inbound_quantity AS split_received_inbound_quantity
FROM inbound i
GROUP BY 1,2,3,4
ORDER BY 1,2,3,4;

I have no clue how to achieve this.
Do you have any idea?

Answer 1

I came up with the following:

select i.campaign, i.expected_inbound_date, i.expected_inbound_quantity, i.received_inbound_quantity, (
    select greatest(
        least(
            i.expected_inbound_quantity, 
            (select sum(iii.received_inbound_quantity) from inbound iii where i.campaign = iii.campaign) - 
            (
                select cum_sum
                from (
                    select sum(ii.expected_inbound_quantity) over (partition by ii.campaign order by ii.expected_inbound_date, ii.expected_inbound_quantity, ii.received_inbound_quantity rows between unbounded preceding and 1 preceding) cum_sum, ii.campaign, ii.expected_inbound_date, ii.expected_inbound_quantity, ii.received_inbound_quantity
                    from inbound ii
                ) tmp
                where (tmp.campaign, tmp.expected_inbound_date, tmp.expected_inbound_quantity, tmp.received_inbound_quantity) = (i.campaign, i.expected_inbound_date, i.expected_inbound_quantity, i.received_inbound_quantity)
            )
        ),
        0
    )
) split
from inbound i
order by i.campaign, i.expected_inbound_date, i.expected_inbound_quantity, i.received_inbound_quantity

Here is a db fiddle.

The idea is to calculate the cumulative sum of rows preceding the current row as cum_sum and then to pick whatever is less: the sum of received_inbound_quantity minus the cum_sum or the expected_inbound_date. To avoid values < 0, I've used greatest.

Answer 2

With reference to the answer in this question an alternative way to solve the issue which is also working in redshift:

DB-Fiddle

SELECT
t1.campaign AS campaign,
t1.expected_inbound_date AS expected_inbound_date,
t1.expected_inbound_quantity AS expected_inbound_quantity,
t1.received_inbound_quantity AS received_inbound_quantity,
t1.quantity_accumulated AS quantity_accumulated,
t1.quantity_total AS quantity_total,


  (CASE WHEN (t1.quantity_total - t1.quantity_accumulated) >=0
  THEN t1.expected_inbound_quantity ELSE
  
    (CASE WHEN (t1.expected_inbound_quantity + t1.quantity_total - t1.quantity_accumulated) >=0
    THEN (t1.expected_inbound_quantity + t1.quantity_total - t1.quantity_accumulated)
    ELSE 0 END)
    
  END) AS split

FROM

  (SELECT
  i.campaign AS campaign,
  i.expected_inbound_date AS expected_inbound_date,
  i.expected_inbound_quantity AS expected_inbound_quantity,
  i.received_inbound_quantity AS received_inbound_quantity,
  SUM(i.expected_inbound_quantity) OVER (PARTITION BY i.campaign ORDER BY i.expected_inbound_date, i.expected_inbound_quantity ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS quantity_accumulated,
  MAX(i.received_inbound_quantity) OVER (PARTITION BY i.campaign) AS quantity_total
  FROM inbound i) t1
  
GROUP BY 1,2,3,4,5,6
ORDER BY 1,2,3,4,5,6;