SQL Server 2019

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SELECT * INTO #data FROM (VALUES
(CAST('2021-08-15' AS DATE),  'Bulgaria', 1081636),
('2021-08-16',  'Bulgaria', 1084693),
('2021-08-17',  'Bulgaria', 1089066),
('2021-08-18',  'Bulgaria', NULL),
('2021-08-19',  'Bulgaria', NULL),
('2021-08-20',  'Bulgaria', 1189066)) x(dt, country, cnt)
6 rows affected

SELECT dt
     , country
     , cnt
     , MAX(cnt) OVER (PARTITION BY country ORDER BY dt)
  FROM #data
 ORDER BY dt


dt
country
cnt
(No column name)


2021-08-15
Bulgaria
1081636
1081636

2021-08-16
Bulgaria
1084693
1084693

2021-08-17
Bulgaria
1089066
1089066

2021-08-18
Bulgaria
null
1089066

2021-08-19
Bulgaria
null
1089066

2021-08-20
Bulgaria
1189066
1189066


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


SELECT dt
     , country
     , cnt
     , SUM(cnt) OVER (PARTITION BY country, partition)
FROM (
    SELECT country
         , dt
         , cnt
         , SUM(CASE WHEN cnt IS NULL THEN 0 ELSE 1 END) OVER (PARTITION BY country ORDER BY dt) AS partition
    FROM #data
) AS d
ORDER BY dt


dt
country
cnt
(No column name)


2021-08-15
Bulgaria
1081636
1081636

2021-08-16
Bulgaria
1084693
1084693

2021-08-17
Bulgaria
1089066
1089066

2021-08-18
Bulgaria
null
1089066

2021-08-19
Bulgaria
null
1089066

2021-08-20
Bulgaria
1189066
1189066


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

dt	country	cnt	(No column name)
2021-08-15	Bulgaria	1081636	1081636
2021-08-16	Bulgaria	1084693	1084693
2021-08-17	Bulgaria	1089066	1089066
2021-08-18	Bulgaria	null	1089066
2021-08-19	Bulgaria	null	1089066
2021-08-20	Bulgaria	1189066	1189066