sql exclude rows based on first occurrence of data and conditions - mysql

Question

I have created a dataset that has columns for 2 customers:
Cust_No Transaction_date amount credit_debit running_total row_num
1 5/27/2022 800 D -200 1
1 5/26/2022 300 D 600 2
1 5/22/2022 800 C 900 3
1 5/20/2022 100 C 100 4
9 5/16/2022 500 D -300 1
9 5/14/2022 300 D 200 2
9 5/6/2022 200 C 500 3
9 5/5/2022 500 D 300 4
9 5/2/2022 300 D 800 5
9 5/2/2022 500 C 1100 6
9 5/1/2022 500 C 600 7
9 5/1/2022 100 C 100 8
The result I am looking for is:
Cust_No Transaction_date amount credit_debit running_total row_num
1 5/27/2022 800 D -200 1
1 5/26/2022 300 D 600 2
1 5/22/2022 800 C 900 3
9 5/16/2022 500 D -300 1
9 5/14/2022 300 D 200 2
9 5/6/2022 200 C 500 3
9 5/5/2022 500 D 300 4
9 5/2/2022 300 D 800 5
9 5/2/2022 500 C 1100 6
I sorted the dataset based on latest transaction for each customer.
We note the latest transaction amount and search for first occurrence of same amount that was a credit (C) and exclude the rest of the rows after it.
In the example above: Customer 9 has lastest debit transaction of 500, so we look for most recent credit transaction of 500 and exclude all the rows after that for customer 9.
Progress Made so far:
calculated the running total using logic:
sum (case when credit_debit ='C' then amount else -1*amount end) over (partition by cust_no order by transaction_date desc ) as running_total
I also got the data using lead 1,2,3,4,5 but this is not efficient and I could have multiple rows before I find the first credit number with amount same as 1st row:
case when lead(amount, 1) over(partition by cust_no order by transaction_date desc) = amount then amount else null end as lead1

Answer 1

No sure which dbms this is for but it need a lateral join in postgres.
It searches for the most recent transaction identified when rn = 1, then it matches that amount to an earlier credit transaction of the same amount and using the rn of that row to form a boundary of row numbers to be returned:
with CTE as (
select
Cust_No, Transaction_date, amount, credit_debit, running_total
, row_number() over(partition by cust_no order by transaction_date DESC) as rn
from mytable
)
, RANGE as (
select *
from CTE
left join lateral (
select c.rn as ignore_after
from CTE as c
where CTE.Cust_No = c.Cust_No
and CTE.amount = c.amount
and c.credit_debit = 'C'
and CTE.rn = 1
order by c.rn ASC
limit 1
) oa on true
where CTE.rn = 1
)
select
CTE.*
from CTE
inner join RANGE on CTE.rn between RANGE.rn and RANGE.ignore_after
and CTE.cust_no = RANGE.cust_no
Cust_No | Transaction_date | amount | credit_debit | running_total | rn
------: | :--------------- | -----: | :----------- | ------------: | -:
1 | 2022-05-27 | 800 | D | -200 | 1
1 | 2022-05-26 | 300 | D | 600 | 2
1 | 2022-05-22 | 800 | C | 900 | 3
9 | 2022-05-16 | 500 | D | -300 | 1
9 | 2022-05-14 | 300 | D | 200 | 2
9 | 2022-05-06 | 200 | C | 500 | 3
9 | 2022-05-05 | 500 | D | 300 | 4
9 | 2022-05-02 | 300 | D | 800 | 5
9 | 2022-05-02 | 500 | C | 1100 | 6
for postgres see: db<>fiddle here
nb: for an "outer apply" example I have also used SQL Server in the following fiddle see: db<>fiddle here