Oracle 18c

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create table test
(
avail_date  date
,size_id    varchar2(10)
,purchases  number
,sales      number
,oh_qty     number
);
insert into test values (to_date('3/1/2024','mm/dd/yyyy'),'30',0,0,20);
1 rows affected

insert into test values (to_date('3/1/2024','mm/dd/yyyy'),'32',0,0,15);
1 rows affected

insert into test values (to_date('3/1/2024','mm/dd/yyyy'),'34',0,5,10);
1 rows affected

insert into test values (to_date('3/25/2024','mm/dd/yyyy'),'30',5,0,20);
1 rows affected

insert into test values (to_date('3/25/2024','mm/dd/yyyy'),'32',0,1,15);
1 rows affected

insert into test values (to_date('3/25/2024','mm/dd/yyyy'),'34',3,5,10);
1 rows affected

insert into test values (to_date('4/5/2024','mm/dd/yyyy'),'30',1,0,20);
1 rows affected

insert into test values (to_date('4/5/2024','mm/dd/yyyy'),'32',0,0,15);
1 rows affected

insert into test values (to_date('4/5/2024','mm/dd/yyyy'),'34',1,1,10);
1 rows affected

select 
  avail_date  
,size_id    
,purchases  
,sales      
,oh_qty  
  , oh_qty + SUM(purchases  - sales) OVER(PARTITION BY size_id ORDER BY avail_date ASC) avail_qty
  from test
order by avail_date, size_id;


AVAIL_DATE
SIZE_ID
PURCHASES
SALES
OH_QTY
AVAIL_QTY


01-MAR-24
30
0
0
20
20

01-MAR-24
32
0
0
15
15

01-MAR-24
34
0
5
10
5

25-MAR-24
30
5
0
20
25

25-MAR-24
32
0
1
15
14

25-MAR-24
34
3
5
10
3

05-APR-24
30
1
0
20
26

05-APR-24
32
0
0
15
14

05-APR-24
34
1
1
10
3

AVAIL_DATE	SIZE_ID	PURCHASES	SALES	OH_QTY	AVAIL_QTY
01-MAR-24	30	0	0	20	20
01-MAR-24	32	0	0	15	15
01-MAR-24	34	0	5	10	5
25-MAR-24	30	5	0	20	25
25-MAR-24	32	0	1	15	14
25-MAR-24	34	3	5	10	3
05-APR-24	30	1	0	20	26
05-APR-24	32	0	0	15	14
05-APR-24	34	1	1	10	3