Subtract a number from the rows until the number ends to zero - mysql

Question

How can subtract a number from the rows to the end of the subtraction in MySQL with query update
If have table like this
Store Table
itemId storeCode qoh
1 1 20
1 2 30
1 3 40
and i want subtract "80" form qoh to get the output
itemId storeCode qoh
1 1 0
1 2 0
1 3 10
I tried by and not work
set #sum = 80;
Update store SET qoh =
(SELECT IF((#sum := #sum - qoh) > 0,0,qoh))
ORDER BY storeCode ASC;
What is the appropriate adjustment to do?

Answer 1

If you are running MySQL 8.0, you can do the computation with window functions.
The following select query gives you the expected results:
select
s.*,
case when sum(qoh) over(partition by itemid order by storecode) - qoh >= 80
then qoh
else greatest(
sum(qoh) over(partition by itemid order by storecode) - 80,
0
)
end new_qoh
from store s
You can then turn this to an update:
update store s
inner join (
select
s.*,
sum(qoh) over(partition by itemid order by storecode) sum_qoh
from store s
) n
on n.itemid = s.itemid
and n.storecode = s.storecode
and n.sum_qoh - s.qoh < 80
set s.qoh = greatest(n.sum_qoh - 80, 0)
Demo on DB Fiddle:
itemId | storeCode | qoh
-----: | --------: | --:
1 | 1 | 0
1 | 2 | 0
1 | 3 | 10
1 | 4 | 50
I added an extra line at the end of your data to demonstrate that the queries leave the "following" qon untouched.