Postgres 15

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CREATE TABLE t
    (saledate date, salesum int)
;
    
INSERT INTO t
    (saledate, salesum)
VALUES
    ('2022-01-01 00:00:00', 100),
    ('2022-01-02 00:00:00', 150),
    ('2022-01-03 00:00:00', 200),
    ('2022-01-05 00:00:00', 100)
;

CREATE TABLE

INSERT 0 4

select *
from   t

saledate
salesum

2022-01-01
100

2022-01-02
150

2022-01-03
200

2022-01-05
100

SELECT 4

select  saledate
       ,salesum + coalesce(lag(salesum) over(order by saledate),0) as salesum
from    t

saledate
salesum

2022-01-01
100

2022-01-02
250

2022-01-03
350

2022-01-05
300

SELECT 4