Oracle SQL recursive adding values - sql

Question

I have the following data in the table
Period Total_amount R_total
01/01/20 2 2
01/02/20 5 null
01/03/20 3 null
01/04/20 8 null
01/05/20 31 null
Based on the above data I would like to have the following situation.
Period Total_amount R_total
01/01/20 2 2
01/02/20 5 3
01/03/20 3 0
01/04/20 8 8
01/05/20 31 23
Additional data
01/06/20 21 0 (previously it would be -2)
01/07/20 25 25
01/08/20 29 4
Pattern to the additional data is:
if total_amount < previous(r_total) then 0
Based on the filled data, we can spot the pattern is:
R_total = total_amount - previous(R_total)
Could you please help me out with this issue?

Answer 1

As Gordon Linoff suspected, it is possible to solve this problem with analytic functions. The benefit is that the query will likely be much faster. The price to pay for that benefit is that you need to do a bit of math beforehand (before ever thinking about "programming" and "computers").
A bit of elementary arithmetic shows that R_TOTAL is an alternating sum of TOTAL_AMOUNT. This can be arranged easily by using ROW_NUMBER() (to get the signs) and then an analytic SUM(), as shown below.
Table setup:
create table sample_data (period, total_amount) as
select to_date('01/01/20', 'mm/dd/rr'), 2 from dual union all
select to_date('01/02/20', 'mm/dd/rr'), 5 from dual union all
select to_date('01/03/20', 'mm/dd/rr'), 3 from dual union all
select to_date('01/04/20', 'mm/dd/rr'), 8 from dual union all
select to_date('01/05/20', 'mm/dd/rr'), 31 from dual
;
Query and result:
with
prep (period, total_amount, sgn) as (
select period, total_amount,
case mod(row_number() over (order by period), 2) when 0 then 1 else -1 end
from sample_data
)
select period, total_amount,
sgn * sum(sgn * total_amount) over (order by period) as r_total
from prep
;
PERIOD TOTAL_AMOUNT R_TOTAL
-------- ------------ ----------
01/01/20 2 2
01/02/20 5 3
01/03/20 3 0
01/04/20 8 8
01/05/20 31 23

Answer 2

This may be possible with window functions, but the simplest method is probably a recursive CTE:
with t as (
select t.*, row_number() over (order by period) as seqnum
from yourtable t
),
cte(period, total_amount, r_amount, seqnum) as (
select period, total_amount, r_amount, seqnum
from t
where seqnum = 1
union all
select t.period, t.total_amount, t.total_amount - cte.r_amount, t.seqnum
from cte join
t
on t.seqnum = cte.seqnum + 1
)
select *
from cte;
This question explicitly talks about "recursively" adding values. If you want to solve this using another mechanism, you might explain the logic in detail and ask if there is a non-recursive CTE solution.