SQL Server 2022

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CREATE TABLE table1 (
    ID INT,
    A DATE,
    AB DATE,
    B DATE,
    C DATE
);

INSERT INTO table1 (ID, A, AB, B, C)
VALUES
    (1, '2021-01-01', '2021-02-01', '2021-03-01', '2021-04-01'),
    (2, NULL, '2021-06-01', NULL, '2021-08-01'),
    (3, '2021-09-01', NULL, '2021-08-01', NULL),
    (4, NULL, NULL, '2021-11-01', NULL),  
    (5, '2021-09-01', '2021-06-01', NULL, NULL),
    (6, '2021-09-01', NULL, '2021-11-01', '2021-11-02'),
    (7, '2021-09-01', '2021-09-01', '2021-11-01', '2021-11-02'),
    (8, '2021-09-01', NULL, '2021-09-01', '2021-11-02'),
    (9, '2021-09-01', NULL, '2021-09-01', '2021-08-02'),
    (10, '2021-09-01', NULL, '2021-09-01', '2021-07-02'),
    (11, '2021-09-01', '2021-09-01', NULL, NULL),
    (12, '2021-08-01', '2021-09-01', NULL, NULL),
    (13, '2021-08-01', '2021-09-01', NULL, '2021-10-01'),
    (14, '2021-08-01', NULL, NULL, '2021-09-01'),
    (15, '2021-09-01', '2021-12-01', NULL, '2021-09-01'),  
    (16, '2021-10-01', NULL, NULL, '2021-09-01')

SELECT *
FROM Table1;

ID	A	AB	B	C
1	2021-01-01	2021-02-01	2021-03-01	2021-04-01
2	null	2021-06-01	null	2021-08-01
3	2021-09-01	null	2021-08-01	null
4	null	null	2021-11-01	null
5	2021-09-01	2021-06-01	null	null
6	2021-09-01	null	2021-11-01	2021-11-02
7	2021-09-01	2021-09-01	2021-11-01	2021-11-02
8	2021-09-01	null	2021-09-01	2021-11-02
9	2021-09-01	null	2021-09-01	2021-08-02
10	2021-09-01	null	2021-09-01	2021-07-02
11	2021-09-01	2021-09-01	null	null
12	2021-08-01	2021-09-01	null	null
13	2021-08-01	2021-09-01	null	2021-10-01
14	2021-08-01	null	null	2021-09-01
15	2021-09-01	2021-12-01	null	2021-09-01
16	2021-10-01	null	null	2021-09-01

ID	A	AB	B	C	SupplementDate
1	2021-01-01	2021-02-01	2021-03-01	2021-04-01	2021-02-01
3	2021-09-01	null	2021-08-01	null	null
4	null	null	2021-11-01	null	null
5	2021-09-01	2021-06-01	null	null	2021-06-01
6	2021-09-01	null	2021-11-01	2021-11-02	2021-11-01
7	2021-09-01	2021-09-01	2021-11-01	2021-11-02	2021-09-01
8	2021-09-01	null	2021-09-01	2021-11-02	2021-09-01
9	2021-09-01	null	2021-09-01	2021-08-02	2021-09-01
10	2021-09-01	null	2021-09-01	2021-07-02	2021-09-01
11	2021-09-01	2021-09-01	null	null	2021-09-01
12	2021-08-01	2021-09-01	null	null	2021-09-01