MySQL 8.0

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CREATE TABLE employees(timestamp datetime);
(SELECT  timestamp FROM employees ORDER BY timestamp DESC LIMIT 1)
UNION ALL
SELECT '1900-01-01 00:00:00.000'
WHERE NOT EXISTS (SELECT 1 FROM employees);

timestamp

1900-01-01 00:00:00.000

SELECT COALESCE(MAX(timestamp), '1900-01-01 00:00:00.000') AS timestamp
FROM employees;

timestamp

1900-01-01 00:00:00.000

INSERT INTO employees VALUES ('2022-09-10 00:00:00'), ('2022-09-11 00:00:00');
(SELECT  timestamp FROM employees ORDER BY timestamp DESC LIMIT 1)
UNION ALL
SELECT '1900-01-01 00:00:00.000'
WHERE NOT EXISTS (SELECT 1 FROM employees);

timestamp

2022-09-11 00:00:00

SELECT COALESCE(MAX(timestamp), '1900-01-01 00:00:00.000') AS timestamp
FROM employees;

timestamp

2022-09-11 00:00:00