SQL Server 2019

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create table trial (id int identity, val int)

INSERT INTO trial (val)
VALUES (1), (0),(0), (0), (0),(0), (7), (0),(9), (0), (0),(12), (0), (14),(0) 

15 rows affected

select (case when max(val) over (order by id) = 0
             then min(case when val > 0 then val end) over (order by id desc)
             else max(val) over (order by id) 
        end) as running_max
from trial
order by id;

running_max

1

1

1

1

1

1

7

7

9

9

9

12

12

14

14

Warning: Null value is eliminated by an aggregate or other SET operation.

truncate table trial
INSERT INTO trial (val)
    VALUES (0), (0),(3), (0), (0),(0), (7), (0),(9), (0), (0),(12), (0), (14),(0) 
15 rows affected

select (case when max(val) over (order by id) = 0
             then min(case when val > 0 then val end) over (order by id desc)
             else max(val) over (order by id) 
        end) as running_max
from trial
order by id;

running_max

3

3

3

3

3

3

7

7

9

9

9

12

12

14

14

Warning: Null value is eliminated by an aggregate or other SET operation.