MySQL 5.7

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CREATE TABLE dt (
  `ID` VARCHAR(7),
  `GROUP` VARCHAR(2),
  `TIMESTAMP1` VARCHAR(23),
  `col_OTHER` VARCHAR(7),
  `TIMESTAMP2` VARCHAR(23)
);

INSERT INTO dt
  (`ID`, `GROUP`, `TIMESTAMP1`, `col_OTHER`, `TIMESTAMP2`)
VALUES
  ('001', 'AA', '2021-04-02 15:02:33.319', 'mangos', null),
  ('002', 'BB', null, 'kiwis', null),
  ('004', 'AA', '2021-04-02 03:51:35.279', 'oranges', null),
  ('003', 'DD', '2021-04-03 18:24:23.469', 'oranges', '2021-04-03 18:22:23.469'),
  ('003', 'DD', null, 'bananas', '2021-04-03 15:02:33.319'),
  ('002', 'CC', '2021-04-04 11:02:51.313', 'apples', '2021-04-04 11:03:51.313');
SELECT * 
FROM dt
WHERE COALESCE(TIMESTAMP1, '1970-01-01') < '2021-04-03'
   OR COALESCE(TIMESTAMP1, '1970-01-01') < TIMESTAMP2


ID
GROUP
TIMESTAMP1
col_OTHER
TIMESTAMP2


001
AA
2021-04-02 15:02:33.319
mangos
null

002
BB
null
kiwis
null

004
AA
2021-04-02 03:51:35.279
oranges
null

003
DD
null
bananas
2021-04-03 15:02:33.319

002
CC
2021-04-04 11:02:51.313
apples
2021-04-04 11:03:51.313


SELECT * 
FROM dt
WHERE COALESCE(TIMESTAMP1, '1970-01-01') < COALESCE(TIMESTAMP2, '2021-04-03')


ID
GROUP
TIMESTAMP1
col_OTHER
TIMESTAMP2


001
AA
2021-04-02 15:02:33.319
mangos
null

002
BB
null
kiwis
null

004
AA
2021-04-02 03:51:35.279
oranges
null

003
DD
null
bananas
2021-04-03 15:02:33.319

002
CC
2021-04-04 11:02:51.313
apples
2021-04-04 11:03:51.313

ID	GROUP	TIMESTAMP1	col_OTHER	TIMESTAMP2
001	AA	2021-04-02 15:02:33.319	mangos	null
002	BB	null	kiwis	null
004	AA	2021-04-02 03:51:35.279	oranges	null
003	DD	null	bananas	2021-04-03 15:02:33.319
002	CC	2021-04-04 11:02:51.313	apples	2021-04-04 11:03:51.313