SQL Server 2022

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CREATE TABLE tblG_L_Entry (
    G_L_EntryNo INT,
    PostingDate DATE,
    Amount DECIMAL(10, 2),
    G_L_AccountNo VARCHAR(10)
);
INSERT INTO tblG_L_Entry (G_L_EntryNo, PostingDate, Amount, G_L_AccountNo)
VALUES 
    (2, '2021-08-01', -15.000, '1010000'),
    (2, '2021-09-01', -525, '1010000'),
    (2, '2021-10-01', -588.000, '1010000'),
    (2, '2021-11-01', -1141.980, '1010000'),
    (2, '2021-12-01', -2174.000, '1010000'),
    (2, '2022-01-01', -21, '1010000'),
    (2, '2022-02-01', -34, '1010000'),
    (2, '2022-03-01', -156, '1010000');
8 rows affected

WITH MonthlyChanges
AS (
    SELECT
          DATEPART(YEAR, PostingDate) AS year
        , DATEPART(MONTH, PostingDate) AS month
        , SUM(CASE 
                WHEN G_L_EntryNo > 1
                    AND PostingDate >= DATEFROMPARTS(DATEPART(YEAR, PostingDate), DATEPART(MONTH, PostingDate), 1)
                    THEN Amount
                ELSE 0
                END) AS change
    FROM tblG_L_Entry
    WHERE G_L_AccountNo = '1010000'
    GROUP BY
          DATEPART(YEAR, PostingDate)
        , DATEPART(MONTH, PostingDate)
    )
SELECT
      year
    , month
    , change
    , SUM(change) OVER (
        ORDER BY year
            , month ROWS UNBOUNDED PRECEDING
        ) AS opening_balance
FROM MonthlyChanges
ORDER BY
      year
    , month;


year
month
change
opening_balance


2021
8
-15.00
-15.00

2021
9
-525.00
-540.00

2021
10
-588.00
-1128.00

2021
11
-1141.98
-2269.98

2021
12
-2174.00
-4443.98

2022
1
-21.00
-4464.98

2022
2
-34.00
-4498.98

2022
3
-156.00
-4654.98

year	month	change	opening_balance
2021	8	-15.00	-15.00
2021	9	-525.00	-540.00
2021	10	-588.00	-1128.00
2021	11	-1141.98	-2269.98
2021	12	-2174.00	-4443.98
2022	1	-21.00	-4464.98
2022	2	-34.00	-4498.98
2022	3	-156.00	-4654.98