Database only Running total with with conditions - postgresql

Question

There are a ton of questions about calculating running totals with Postgres but I am struggling to do something slightly different.
I have a table that looks like this
txn_id
amount
String
Integer
amounts can either be positive or negative.
I am trying to return a table that looks like this
txn_id
amount
running_total
overage_total
String
Integer
Integer
Integer
Where running total is running sum of the amount column as long as the amount is greater than zero and overage_total is the running sum of amounts that were lower than zero.
An example of would be
txn_id
amount
a
1
b
2
c
-4
d
2
e
-1
I have been using a window function for the running sum but it's not quite what we need.
The correct table would return
txn_id
amount
running_total
overage_total
a
1
1
0
b
2
3
0
c
-4
0
1
d
2
2
1
e
-1
1
1
Currently I have am doing this in code but it would be really incredible to do it in the database if it's possible.

Answer 1

The pattern here is running total with a cap. It could be achieved with recursive cte:
WITH RECURSIVE cte_r AS (
SELECT t.*, ROW_NUMBER() OVER(ORDER BY t.txn_id) AS rn FROM tab t
), cte AS (
SELECT rn,
txn_id,
amount,
CASE WHEN amount <= 0 THEN 0 ELSE amount END AS total,
CASE WHEN amount <= 0 THEN 1 ELSE 0 END AS overage_total
FROM cte_r
WHERE rn = 1
UNION ALL
SELECT cte_r.rn,
cte_r.txn_id,
cte_r.amount,
CASE WHEN cte.total + cte_r.amount <= 0 THEN 0
ELSE cte.total + cte_r.amount
END AS total,
cte.overage_total + CASE WHEN cte.total + cte_r.amount <= 0
THEN 1 ELSE 0 END AS overage_total
FROM cte
JOIN cte_r
ON cte.rn = cte_r.rn-1
)
SELECT txn_id, amount, total,overage_total
FROM cte
ORDER BY rn;
Output:
+---------+---------+--------+---------------+
| txn_id | amount | total | overage_total |
+---------+---------+--------+---------------+
| a | 1 | 1 | 0 |
| b | 2 | 3 | 0 |
| c | -4 | 0 | 1 |
| d | 2 | 2 | 1 |
| e | -1 | 1 | 1 |
| f | 2 | 3 | 1 |
| h | -4 | 0 | 2 |
+---------+---------+--------+---------------+
db<>fiddle demo
Related: Conditional SUM on Oracle and 7. Capping a running total

Answer 2

An option is to use a function to step through the rows and do calculations:
CREATE FUNCTION runningTotalWithCondition() RETURNS TABLE(txn_id char(1), amount int, running_total integer, overage_total integer) AS
$$
DECLARE
running_total integer := 0;
overage_total integer := 0;
c CURSOR FOR SELECT * FROM t ORDER BY txn_id ASC;
BEGIN
FOR recordvar IN c LOOP
IF (running_total + recordvar.amount) > 0 THEN
running_total = running_total + recordvar.amount;
overage_total = overage_total;
ELSE
overage_total = overage_total + abs(running_total + recordvar.amount);
running_total = 0;
END IF;
RETURN QUERY SELECT recordvar.txn_id, recordvar.amount, running_total, overage_total;
END LOOP;
END;
$$ LANGUAGE plpgsql;
Calling the function:
SELECT * FROM runningTotalWithCondition();