Postgres Bank Account Transaction Balance - sql

Question

Here's an example "transactions" table where each row is a record of an amount and the date of the transaction.
+--------+------------+
| amount | date |
+--------+------------+
| 1000 | 2020-01-06 |
| -10 | 2020-01-14 |
| -75 | 2020-01-20 |
| -5 | 2020-01-25 |
| -4 | 2020-01-29 |
| 2000 | 2020-03-10 |
| -75 | 2020-03-12 |
| -20 | 2020-03-15 |
| 40 | 2020-03-15 |
| -50 | 2020-03-17 |
| 200 | 2020-10-10 |
| -200 | 2020-10-10 |
+--------+------------+
The goal is to return one column "balance" with the balance of all transactions. Only catch is that there is a monthly fee of $5 for each month that there are not at least THREE payment transactions (represented by a negative value in the amount column) that total at least $100. So in the example, the only month where you wouldn't have a $5 fee is March because there were 3 payments (negative amount transactions) that totaled $145. So the final balance would be $2,746. The sum of the amounts is $2,801 minus the $55 monthly fees (11 months X 5). I'm not a postgres expert by any means, so if anyone has any pointers on how to get started solving this problem or what parts of the postgres documentation which help me most with this problem that would be much appreciated.
The expected output would be:
+---------+
| balance |
+---------+
| 2746 |
+---------+

Answer 1

This is rather complicated. You can calculate the total span of months and then subtract out the one where the fee is cancelled:
select amount, (extract(year from age) * 12 + extract(month from age)), cnt,
amount - 5 *( extract(year from age) * 12 + extract(month from age) + 1 - cnt) as balance
from (select sum(amount) as amount,
age(max(date), min(date)) as age
from transactions t
) t cross join
(select count(*) as cnt
from (select date_trunc('month', date) as yyyymm, count(*) as cnt, sum(amount) as amount
from transactions t
where amount < 0
group by yyyymm
having count(*) >= 3 and sum(amount) < -100
) tt
) tt;
Here is a db<>fiddle.
This calculates 2756, which appears to follow your rules. If you want the full year, you can just use 12 instead of the calculating using the age().

Answer 2

I would first left join with a generate_series that represents the months you are interested in (in this case, all in the year 2020). That adds the missing months with a balance of 0.
Then I aggregate these values per month and add the negative balance per month and the number of negative balances.
Finally, I calculate the grand total and subtract the fee for each month that does not meet the criteria.
SELECT sum(amount_per_month) -
sum(5) FILTER (WHERE negative_per_month > -100 OR negative_count < 3)
FROM (SELECT sum(amount) AS amount_per_month,
sum(amount) FILTER (WHERE amount < 0) AS negative_per_month,
month_start,
count(*) FILTER (WHERE amount < 0) AS negative_count
FROM (SELECT coalesce(t.amount, 0) AS amount,
coalesce(date_trunc('month', CAST (t.day AS timestamp)), dates.d) AS month_start
FROM generate_series(
TIMESTAMP '2020-01-01',
TIMESTAMP '2020-12-01',
INTERVAL '1 month'
) AS dates (d)
LEFT JOIN transactions AS t
ON dates.d = date_trunc('month', CAST (t.day AS timestamp))
) AS gaps_filled
GROUP BY month_start
) AS sums_per_month;

Answer 3

This would be my solution by simply using cte.
DB fiddle here.
balance
2746
Code:
WITH monthly_credited_transactions
AS (SELECT Date_part('month', date) AS cred_month,
Sum(CASE
WHEN amount < 0 THEN Abs(amount)
ELSE 0
END) AS credited_amount,
Sum(CASE
WHEN amount < 0 THEN 1
ELSE 0
END) AS credited_cnt
FROM transactions
GROUP BY 1),
credit_fee
AS (SELECT ( 12 - Count(1) ) * 5 AS fee,
1 AS id
FROM monthly_credited_transactions
WHERE credited_amount >= 100
AND credited_cnt >= 3),
trans
AS (SELECT Sum(amount) AS amount,
1 AS id
FROM transactions)
SELECT amount - fee AS balance
FROM trans a
LEFT JOIN credit_fee b
ON a.id = b.id

Answer 4

For me the below query worked (have adopted my answer from #GordonLinoff):
select CAST(totalamount - 5 *(12 - extract(month from firstt) + 1 - nofeemonths) AS int) as balance
from (select sum(amount) as totalamount, min(date) as firstt
from transactions t
) t cross join
(select count(*) as nofeemonths
from (select date_trunc('month', date) as months, count(*) as nofeemonths, sum(amount) as totalamount
from transactions t
where amount < 0
group by months
having count(*) >= 3 and sum(amount) < -100
) tt
) tt;
The firstt is the date of first transaction in that year and 12 - extract(month from firstt) + 1 - nofeemonths are the number of months for which the credit card fees of 5 will be charged.