SQL Server 2019

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CREATE TABLE LoadData ([Sequence_Code] varchar(4), [ID] int, [Current_Year] int, [Record_Date] date);
    
INSERT INTO LoadData ([Sequence_Code], [ID], [Current_Year], [Record_Date]) VALUES
    (NULL, 310001, 2018, '2018-01-01'),
    (NULL, 310002, 2019, '2019-02-22'),
    (NULL, 310003, 2020, '2020-02-20'),
    (NULL, 310004, 2020, '2020-02-10');
    
SELECT * FROM LoadData;  


Sequence_Code
ID
Current_Year
Record_Date


null
310001
2018
2018-01-01

null
310002
2019
2019-02-22

null
310003
2020
2020-02-20

null
310004
2020
2020-02-10


WITH cte AS (SELECT *, ROW_NUMBER() OVER (ORDER BY Record_Date) rn FROM LoadData)
SELECT RIGHT(Current_Year, 2) + 
       CHAR(ASCII('A') + rn / 26 + CASE rn % 26 WHEN 0 THEN -1 ELSE 0 END) + 
       CHAR(ASCII('A') - 1 + CASE rn % 26 WHEN 0 THEN 26 ELSE rn % 26 END) Sequence_Code,
       ID, Current_Year, Record_Date
FROM cte
ORDER BY rn


Sequence_Code
ID
Current_Year
Record_Date


18AA
310001
2018
2018-01-01

19AB
310002
2019
2019-02-22

20AC
310004
2020
2020-02-10

20AD
310003
2020
2020-02-20


WITH cte AS (SELECT *, ROW_NUMBER() OVER (ORDER BY Record_Date) rn FROM LoadData)
UPDATE cte
SET Sequence_Code = RIGHT(Current_Year, 2) + 
                    CHAR(ASCII('A') + rn / 26 + CASE rn % 26 WHEN 0 THEN -1 ELSE 0 END) + 
                    CHAR(ASCII('A') - 1 + CASE rn % 26 WHEN 0 THEN 26 ELSE rn % 26 END)
4 rows affected

SELECT * FROM LoadData


Sequence_Code
ID
Current_Year
Record_Date


18AA
310001
2018
2018-01-01

19AB
310002
2019
2019-02-22

20AD
310003
2020
2020-02-20

20AC
310004
2020
2020-02-10

Sequence_Code	ID	Current_Year	Record_Date
null	310001	2018	2018-01-01
null	310002	2019	2019-02-22
null	310003	2020	2020-02-20
null	310004	2020	2020-02-10

Sequence_Code	ID	Current_Year	Record_Date
18AA	310001	2018	2018-01-01
19AB	310002	2019	2019-02-22
20AC	310004	2020	2020-02-10
20AD	310003	2020	2020-02-20