SQL Server 2022

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

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'),
    (17, '2020-12-01', '2022-01-02', '2021-03-12', '2022-09-01')  


  

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

17
2020-12-01
2022-01-02
2021-03-12
2022-09-01


SELECT *,
    CASE
      WHEN (LEAST(AB, B) <= C OR C IS NULL)
       AND (LEAST(AB, A) <= B OR B IS NULL)
      THEN LEAST(AB, B)
    END AS SupplementDate
FROM table1
ORDER BY ID;


ID
A
AB
B
C
SupplementDate


1
2021-01-01
2021-02-01
2021-03-01
2021-04-01
2021-02-01

2
null
2021-06-01
null
2021-08-01
2021-06-01

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
null

10
2021-09-01
null
2021-09-01
2021-07-02
null

11
2021-09-01
2021-09-01
null
null
2021-09-01

12
2021-08-01
2021-09-01
null
null
2021-09-01

13
2021-08-01
2021-09-01
null
2021-10-01
2021-09-01

14
2021-08-01
null
null
2021-09-01
null

15
2021-09-01
2021-12-01
null
2021-09-01
null

16
2021-10-01
null
null
2021-09-01
null

17
2020-12-01
2022-01-02
2021-03-12
2022-09-01
2021-03-12


SELECT *,
    CASE
      WHEN (IIF(AB IS NOT NULL, IIF(B IS NOT NULL AND B < AB, B, AB), B) <= C OR C IS NULL)
       AND (IIF(AB IS NOT NULL, IIF(A IS NOT NULL AND A < AB, A, AB), A) <= B OR B IS NULL)
      THEN IIF(AB IS NOT NULL, IIF(B IS NOT NULL AND B < AB, B, AB), B)
    END AS SupplementDate
FROM table1
ORDER BY ID;


ID
A
AB
B
C
SupplementDate


1
2021-01-01
2021-02-01
2021-03-01
2021-04-01
2021-02-01

2
null
2021-06-01
null
2021-08-01
2021-06-01

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
null

10
2021-09-01
null
2021-09-01
2021-07-02
null

11
2021-09-01
2021-09-01
null
null
2021-09-01

12
2021-08-01
2021-09-01
null
null
2021-09-01

13
2021-08-01
2021-09-01
null
2021-10-01
2021-09-01

14
2021-08-01
null
null
2021-09-01
null

15
2021-09-01
2021-12-01
null
2021-09-01
null

16
2021-10-01
null
null
2021-09-01
null

17
2020-12-01
2022-01-02
2021-03-12
2022-09-01
2021-03-12

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
17	2020-12-01	2022-01-02	2021-03-12	2022-09-01