Postgresql – Calculating a cumulative sum with an alias in postgresql

postgresqlwindow functions

I have two tables – user credit and payments.

credit is just a total of the amount of credit a user has at the present.
Payments is a list of payments by date that a user needs to make.

I need to create a report showing the amount of credit available for each payment a user is to make – taking into account the credit used up on previous payments.

CREATE TABLE credit (
  user_id INT,
  amount INT
);

CREATE TABLE payments (
  user_id INT,
  due timestamp,
  amount INT
);

INSERT INTO credit values (1, 100);
INSERT INTO credit values (2, 200);

INSERT INTO payments values (1, '2021-04-01', 20);
INSERT INTO payments values (1, '2021-04-02', 20);
INSERT INTO payments values (1, '2021-04-03', 20);
INSERT INTO payments values (2, '2021-04-01', 100);
INSERT INTO payments values (2, '2021-04-02', 300);
INSERT INTO payments values (3, '2021-04-03', 20);

Result should look like this:

user_id due amount credit_available credit_used credit_remaining
1 2021-04-01 20 100 20 80
1 2021-04-02 20 80 20 60
1 2021-04-02 20 60 20 40
2 2021-04-01 100 200 100 100
2 2021-04-01 200 100 100 0
3 2021-04-01 20 0 0 0

I have set up a fiddle here:

http://sqlfiddle.com/#!17/e812b/9/0

I thought I could do it pretty simply with a LAG() but I can't reference the previous row's calculated alias credit_remaining column within the select like so:

SELECT
    ...,
    least(0, payments.amount - LAG(credit_remaining, 1, credit.amount) OVER (PARTITION BY user_id ORDER BY user_id, due)) as credit_remaining
FROM ...

Best Answer

SELECT user_id, 
       payments.due, 
       payments.amount, 
       COALESCE(credit.amount, 0) + payments.amount - SUM(payments.amount) OVER (PARTITION BY user_id ORDER BY payments.due) available,
       LEAST(payments.amount, COALESCE(credit.amount, 0) + payments.amount - SUM(payments.amount) OVER (PARTITION BY user_id ORDER BY payments.due)) used,
       GREATEST(0, COALESCE(credit.amount, 0) - SUM(payments.amount) OVER (PARTITION BY user_id ORDER BY payments.due)) remaining
FROM ( SELECT user_id FROM credit
       UNION 
       SELECT user_id FROM payments ) userlist
LEFT JOIN credit USING (user_id)
LEFT JOIN payments USING (user_id)
ORDER BY user_id, due

https://dbfiddle.uk/?rdbms=postgres_9.6&fiddle=7e5814dbe02ced69684d331b277dd205

PS. payments (user_id, due) must be defined as unique - if not then the output is indefinite (or you must use another ordering in the window definition, for example, by payments.id additionally).

Related Question