Dividing a sum value into multiple rows due to field length constraint - sql

Question

I am migrating financial data from a very large table (100 million+ of rows) by summarizing the amount and insert them into summary table. I ran into problem when the summary amount (3 billions) is larger than what the field in the summary table can hold (can only hold up to 999 millions.) Changing the field size is not an option as it requires a change process.
The only option I have is to divide the amount (the one that breach the size limit) into smaller ones so it can be inserted into the table.
I came across this SQL - I need to divide a total value into multiple rows in another table which is similar except the number of rows I need to insert is dynamic.
For simplicity, this is how the source table might look like
account_table
acct_num | amt
-------------------------------
101 125.00
101 550.00
101 650.00
101 375.00
101 475.00
102 15.00
103 325.00
103 875.00
104 200.00
104 275.00
The summary records are as follows
select acct_num, sum(amt)
from account_table
group by acct_num
Account Summary
acct_num | amt
-------------------------------
101 2175.00
102 15.00
103 1200.00
104 475.00
Assuming the maximum value in the destination table is 1000.00, the expected output will be
summary_table
acct_num | amt
-------------------------------
101 1000.00
101 1000.00
101 175.00
102 15.00
103 1000.00
103 200.00
104 475.00
How do I create a query to get the expected result? Thanks in advance.

Answer 1

You need a numbers table. If you have a handful of values, you can define it manually. Otherwise, you might have one on hand or use a similar logic:
with n as (
select (rownum - 1) as n
from account_table
where rownum <= 10
),
a as (
select acct_num, sum(amt) as amt
from account_table
group by acct_num
)
select acct_num,
(case when (n.n + 1) * 1000 < amt then 1000
else amt - n.n * 1000
end) as amt
from a join
n
on n.n * 1000 < amt ;

Answer 2

A variation along these lines might give some ideas (using the 1,000 of your sample data):
WITH summary AS (
SELECT acct_num
,TRUNC(SUM(amt) / 1000) AS times
,MOD(SUM(amt), 1000) AS remainder
FROM account_table
GROUP BY acct_num
), x(acct_num, times, remainder) AS (
SELECT acct_num, times, remainder
FROM summary
UNION ALL
SELECT s.acct_num, x.times - 1, s.remainder
FROM summary s
,x
WHERE s.acct_num = x.acct_num
AND x.times > 0
)
SELECT acct_num
,CASE WHEN times = 0 THEN remainder ELSE 1000 END AS amt
FROM x
ORDER BY acct_num, amt DESC
The idea is to first build a summary table with div and modulo:
ACCT_NUM TIMES REMAINDER
101 2 175
102 0 15
103 1 200
104 0 475
Then perform a hierarchical query on the summary table based on the number of "times" (i.e. rows) you want, with an extra for the remainder.
ACCT_NUM AMT
101 1000
101 1000
101 175
102 15
103 1000
103 200
104 475