SQL Server 2017

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CREATE TABLE tablename (Date DATE, Value INTEGER);

INSERT INTO tablename (Date, Value) VALUES
  ('2020-12-27', '2'),
  ('2020-12-28', '8'),
  ('2021-01-01', '1'),
  ('2021-01-02', '10'),
  ('2021-01-28', '7'),
  ('2021-01-27', '11'),
  ('2021-02-10', '12'),
  ('2021-02-07', '5'),
  ('2021-02-11', '8'),
  ('2021-02-04', '2'),
  ('2021-03-17', '5'),
  ('2021-03-03', '0'),
  ('2021-03-12', '15'),
  ('2021-03-12', '11'),
  ('2021-04-11', '5'),
  ('2021-04-26', '7');
16 rows affected

WITH cte AS (
  SELECT CASE 
           WHEN DAY(Date) >= 20 THEN Date 
           ELSE DATEADD(mm, -1, Date) 
         END Date,
         Value
  FROM tablename 
)
SELECT YEAR(Date) year, MONTH(Date) month, SUM(Value) val
FROM cte
GROUP BY YEAR(Date), MONTH(Date)
ORDER BY YEAR(Date), MONTH(Date)


year
month
val


2020
12
21

2021
1
45

2021
2
31

2021
3
5

2021
4
7


WITH cte AS (
  SELECT CASE 
           WHEN DAY(Date) < 20 THEN Date 
           ELSE DATEADD(mm, 1, Date) 
         END Date,
         Value
  FROM tablename 
)
SELECT YEAR(Date) year, MONTH(Date) month, SUM(Value) val
FROM cte
GROUP BY YEAR(Date), MONTH(Date)
ORDER BY YEAR(Date), MONTH(Date)


year
month
val


2021
1
21

2021
2
45

2021
3
31

2021
4
5

2021
5
7


SELECT FORMAT(DATEADD(dd, -20, Date), 'yyyy-MM') year_month, 
       SUM(Value) val
FROM tablename
GROUP BY FORMAT(DATEADD(dd, -20, Date), 'yyyy-MM')


year_month
val


2020-12
21

2021-01
45

2021-02
31

2021-03
5

2021-04
7