MySQL 8.0

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SELECT VERSION();


VERSION()


8.0.30


DROP TABLE IF EXISTS `tbl_test`;

CREATE TABLE IF NOT EXISTS `tbl_test` (
  `datetime` DATETIME NOT NULL,
  `date` DATE NOT NULL,
  `time` TIME NOT NULL
);

INSERT INTO `tbl_test` (
  `datetime`,
  `date`,
  `time`
) VALUES (
  '2020-12-20 16:42:27',
  '2020-12-20',
  '16:42:27'
);
SET @`datetime` := '2020-12-20 16:42:27';

SELECT
  `datetime`,
  DATE_FORMAT(`datetime`, '%Y-%m-%d %H') `a`,
  DATE_FORMAT(@`datetime`, '%Y-%m-%d %H') `b`,
  DATE_FORMAT('2020-12-20 16:42:27', '%Y-%m-%d %H') `c`,
  UNIX_TIMESTAMP(DATE_FORMAT(`datetime`, '%Y-%m-%d %H')) `d`,
  UNIX_TIMESTAMP(DATE_FORMAT(@`datetime`, '%Y-%m-%d %H')) `e`,
  UNIX_TIMESTAMP(DATE_FORMAT('2020-12-20 16:42:27', '%Y-%m-%d %H')) `f`,
  UNIX_TIMESTAMP(DATE_FORMAT(CONCAT('2020-12-20', SPACE(1), '16:42:27'), '%Y-%m-%d %H')) `g`,
  UNIX_TIMESTAMP(DATE_FORMAT(CONCAT(`date`, SPACE(1), `time`), '%Y-%m-%d %H')) `h`
FROM
  `tbl_test`;


datetime
a
b
c
d
e
f
g
h


2020-12-20 16:42:27
2020-12-20 16
2020-12-20 16
2020-12-20 16
1608480000.000000
1608480000.000000
1608480000
1608480000
1608480000.000000