is there way to apply row updates without looping - sql

Question

i would like to pay for different invoices using different credits i have.
drop table #InvoicesWithBalances
drop table #AvailableCredits
create table #InvoicesWithBalances
(
InvoiceKey decimal(18,0) not null,
APBalance decimal(18,6) null,
BalanceAfterCreditApplied decimal(18,6) null,
)
create table #AvailableCredits
(
credit_id int identity(1,1),
StartingBalance decimal(18,6) null,
CurrentBalance decimal(18,6) null,
)
insert into #InvoicesWithBalances values (5452, 13744.080000, 13744.080000)
insert into #InvoicesWithBalances values (7056, 13744.080000, 13744.080000)
insert into #InvoicesWithBalances values (7438, 500.000000, 500.000000 )
insert into #AvailableCredits values ( -13744.080000, -13744.080000)
insert into #AvailableCredits values ( -13700.080000, -13700.080000)
insert into #AvailableCredits values ( -500.000000, -500.000000)
insert into #AvailableCredits values ( -500.000000, -500.000000)
select * from #InvoicesWithBalances
select * from #AvailableCredits
If I was doing a looping solution I would take the largest credit and start applying it to the invoices in order of largest to smallest until the balance of the credit is zero, then I would move on to the next to the next credit until I had no credits and no invoices left.
In the example below the first 2 credits should be fully used. The third credit should be partially used and the last credit should go untouched
Any advice?

Answer 1

I have tried to simulate your example here:
create table InvoicesWithBalances
(
InvoiceKey int not null,
APBalance int null,
BalanceAfterCreditApplied int null,
);
create table AvailableCredits
(
credit_id int identity(1,1),
StartingBalance int null,
CurrentBalance int null,
);
insert into InvoicesWithBalances values (5452, 13744, 13744);
insert into InvoicesWithBalances values (7056, 13744, 13744);
insert into InvoicesWithBalances values (7438, 500, 500);
insert into AvailableCredits values ( -13744, -13744);
insert into AvailableCredits values ( -13700, -13700);
insert into AvailableCredits values ( -500, -500);
insert into AvailableCredits values ( -500, -500);
create table #invoice (invoice_row_num int, InvoiceKey int, APBalance int, BalanceAfterCreditApplied int);
insert into #invoice
select ROW_NUMBER() OVER (ORDER BY APBalance desc) as row_num, InvoiceKey, APBalance, BalanceAfterCreditApplied FROM InvoicesWithBalances;
create table #credits (credit_row_num int, StartingBalance int, CurrentBalance int);
insert into #credits
select ROW_NUMBER() OVER (ORDER BY StartingBalance asc) as row_num, StartingBalance, CurrentBalance FROM AvailableCredits;
create table #invoice_credit_list (invoice_credit_row_num int, init_invoice int, init_credit int);
if ((select max(invoice_row_num) from #invoice) > (select max(credit_row_num) from #credits))
insert into #invoice_credit_list
select i.invoice_row_num , i.APBalance, (-isnull(c.StartingBalance,0)) from
#invoice i
left join
#credits c
on
i.invoice_row_num = c.credit_row_num;
else
insert into #invoice_credit_list
select c.credit_row_num, isnull(i.APBalance,0), (-c.StartingBalance) from
#credits c
left join
#invoice i
on
i.invoice_row_num = c.credit_row_num;
with cte as
(
select
invoice_credit_row_num,
init_invoice,
init_credit,
case when init_invoice >= init_credit then
init_invoice - init_credit
else
0
end as 'invoice_remaining',
case when init_credit >= init_invoice then
init_credit - init_invoice
else
0
end as 'credit_remaining'
from
#invoice_credit_list i
where
i.invoice_credit_row_num = 1
UNION ALL
select
i.invoice_credit_row_num,
i.init_invoice + cte.invoice_remaining as 'init_invoice',
i.init_credit + cte.credit_remaining as 'init credit',
case when (i.init_invoice + cte.invoice_remaining) >= (i.init_credit + cte.credit_remaining ) then
(i.init_invoice + cte.invoice_remaining) - (i.init_credit + cte.credit_remaining )
else
0
end as 'invoice_remaining',
case when (i.init_credit + cte.credit_remaining) >= (i.init_invoice + cte.invoice_remaining) then
(i.init_credit + cte.credit_remaining) - (i.init_invoice + cte.invoice_remaining)
else
0
end as 'credit_remaining'
from
#invoice_credit_list i
inner join
cte
ON
i.invoice_credit_row_num - 1 = cte.invoice_credit_row_num
AND
i.invoice_credit_row_num > 1
)
select * from cte;
and you can also find this simulation with output here: https://rextester.com/SJYGV76640
The 'cte' table in the simulation will give you all the details.
Eventhough, this is now done in db side, I am not sure of its performance. So, please compare and evaluate its performance.
Note:
if this is performing faster, well and good. But, at most, don't opt for loops in T-SQL.
If this is not performing better, go for loops in any other programming language like C#, VB,.. if that is possible.
If no other option works for you, go for loops in T-SQL. But, with increasing data, I am not sure how the server will react :(
Hope this helps you :)