MySQL 8.0

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CREATE TABLE sample_table (
id int,
date_activated date
)
INSERT INTO sample_table
VALUES
(1, '2016-05-12'),
(2, '2019-05-16')
SELECT * FROM sample_table


id
date_activated


1
2016-05-12

2
2019-05-16


ALTER TABLE sample_table
ADD COLUMN datetime timestamp AFTER date_activated;
SELECT * FROM sample_table


id
date_activated
datetime


1
2016-05-12
null

2
2019-05-16
null


SELECT id, 
       date_activated, 
       CONCAT(YEAR(date_activated), '-01-01 00:00:00') AS datetime
FROM sample_table


id
date_activated
datetime


1
2016-05-12
2016-01-01 00:00:00

2
2019-05-16
2019-01-01 00:00:00


UPDATE sample_table a
INNER JOIN sample_table b ON a.id = b.id
SET b.datetime = CONCAT(YEAR(b.date_activated), '-01-01 00:00:00')
SELECT * FROM sample_table


id
date_activated
datetime


1
2016-05-12
2016-01-01 00:00:00

2
2019-05-16
2019-01-01 00:00:00

id	date_activated	datetime
1	2016-05-12	2016-01-01 00:00:00
2	2019-05-16	2019-01-01 00:00:00