MySQL 5.6

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

CREATE TABLE table1 (`datetime` DATETIME)
INSERT INTO table1 VALUEs ('2021-01-01 00:00:00')
,('2021-01-02 00:00:00'),('2021-01-03 00:00:00'),('2021-01-04 00:00:00')
,('2021-01-04 00:00:00'),('2021-01-05 00:00:00'),('2021-01-06 00:00:00'),('2021-01-07 00:00:00'),
('2021-01-08 00:00:00'),('2021-01-09 00:00:00')
SELECT YEAR(`datetime`) AS Year
, DATE_FORMAT(`datetime`, '%b %v') AS Week
, COUNT(*) AS total
FROM table1
GROUP BY Year, Week; 


Year
Week
total


2021
Jan 01
7

2021
Jan 53
3


SELECT 
    year,
    DATE_FORMAT(STR_TO_DATE(CONCAT(year, ' ', week, ' Sunday'), '%X %V %W'), '%b %e') AS week,
    total
FROM
    (SELECT 
        YEAR(`datetime`) AS year,
            WEEK(`datetime`) AS week,
            COUNT(*) AS total
    FROM
        table1
    GROUP BY `year`, `week`) AS sub;


year
week
total


2021
null
2

2021
Jan 3
8