MySQL 8.0

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CREATE TABLE  test
(
  tm time 
);

insert into test values 
('10:00:00'),
('08:00:00'),
('15:00:00'),
('20:00:00'),
('21:00:00');

Records: 5  Duplicates: 0  Warnings: 0

SELECT * 
FROM `test` 
WHERE hour(tm) >= '20' or  hour(tm) <=  '08';

tm

08:00:00

20:00:00

21:00:00

select * from test 
WHERE DATE_FORMAT(tm, '%H')>='20'
      or  DATE_FORMAT(tm, '%H')<='08'

tm

08:00:00

20:00:00

21:00:00