oracle sql running total range - sql

Question

I have two tables tab_a as
SUB_ID AMOUNT
1 10
2 5
3 7
4 15
5 4
2 table tab_b as
slab_number slab_start slab_end
1 12 20
2 21 25
3 26 35
slab_start will always be 1 more than slab_end of previous slab number
If I run the running total for tab_a my result is
select sub_id , sum(amount) OVER(ORDER BY sub_id) run_sum
from tab_a
sub_id run_sum
1 10
2 15
3 22
4 37
5 41
I need to SQL query to check which slab_NUMBER if run_sum is less than first slab_number from then it should be Zero , if run_sum is more than last slab number then blank except the row which crosses the limit .
Expected result is
sub_id run_sum slab_number
1 10 0
2 15 1
3 22 2
4 37 3
5 41 NULL
I have tried this .
First find the running sum which crosses the limit i. e last slab_end
select min( run_sum )
from (select sub_id , sum(amount) OVER(ORDER BY sub_id) run_sum
from tab_a ) where run_sum>=35
then use below query
select sub_id,
run_sum,
case
when run_sum <
(select SLAB_START from tab_b where slab_number = '1') then
0
when run_sum = 37 then
(select max(slab_number) from tab_b)
when run_sum > 37 then
NULL
else
(select slab_number
from tab_b
where run_sum between SLAB_START and slab_end)
end slab_number
from (select sub_id, sum(amount) OVER(ORDER BY sub_id) run_sum from tab_a)
is there any other way to improve.

Answer 1

Somewhat strange requirement :) Use some analytic functions and case when's. Row_number when you need to find something first, max() over() and sum() over() when you need information from over rows:
with
a as (
select sub_id, row_number() over (order by sub_id) rn,
sum(amount) over (order by sub_id) rs
from tab_a),
b as (select tab_b.*, max(slab_number) over () msn from tab_b )
select sub_id, rs,
case when sn is null and row_number() over (partition by sn order by sub_id) = 1
then msn else sn
end sn
from (
select sub_id, rs, max(msn) over () msn,
case when slab_number is null and rn = 1 then 0 else slab_number end sn
from a left join b on rs between slab_start and slab_end)
dbfiddle demo

Answer 2

you could try this:
select a.sub_id , sum(a.amount) OVER(ORDER BY a.sub_id) run_sum
,case when b.slab_number=1 then 0 else lag(b.slab_number,1) over (order by a.sub_id)end slab_number
from tab_a a
left join tab_b b on a.SUB_ID = b.slab_number

Answer 3

I think this is basically a left join with a default value:
select a.*,
(case when a.run_sum < bb.min_slab_num then 0
else b.slab_num
end) as slab_num
from (select sub_id,
sum(amount) over (order by sub_id) as run_sum
from tab_a
) a left join
tab_b b
on a.run_sum between slab_start and slab_end cross join
(select min(slab_start) as min_slab_start
from tab_b
) bb;