MySQL 8.0

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SELECT VERSION();


VERSION()


8.0.27


-- the table and initial data
CREATE TABLE test (id INT, 
                   ts TIMESTAMP);
INSERT INTO test (id, ts) VALUES (1,'2022-01-01'), (2, NULL), (NULL,'2022-01-01');
-- the trigger which should provide (id, ts) uniqueness
-- including NULL values
CREATE TRIGGER tr_bi_check_uniqueness_with_nulls
BEFORE INSERT
ON test
FOR EACH ROW
BEGIN
    IF EXISTS ( SELECT NULL
                FROM test
                WHERE test.id <=> NEW.id
                  AND test.ts <=> NEW.ts ) THEN
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Duplicated values not allowed.';
    END IF;
END
-- try to insert duplicate for 1st row - blocked
INSERT INTO test (id, ts) VALUES (1,'2022-01-01');
Duplicated values not allowed.

-- try to insert duplicate for 2nd row - allowed (???)
INSERT INTO test (id, ts) VALUES (2, NULL);
-- try to insert duplicate for 3rd row - blocked
INSERT INTO test (id, ts) VALUES (NULL,'2022-01-01');
Duplicated values not allowed.

-- check final data state - duplicate detected
SELECT * FROM test;


id
ts


1
2022-01-01 00:00:00

2
null

null
2022-01-01 00:00:00

2
null