MySQL 8.0

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CREATE TABLE test (id INT AUTO_INCREMENT PRIMARY KEY, 
                   category INT, 
                   value INT,
                   UNIQUE (category, value) );
CREATE TRIGGER tr_ai
AFTER INSERT ON test
FOR EACH ROW 
SET @ids_array := CONCAT_WS(',', @ids_array, NEW.id);
CREATE TRIGGER tr_au
AFTER UPDATE ON test
FOR EACH ROW 
SET @ids_array := CONCAT_WS(',', @ids_array, NEW.id);
SET @ids_array := NULL;
INSERT INTO test (category, value)
VALUES (1,11), (2,22);
SELECT * FROM test;
SELECT @ids_array;


id
category
value


1
1
11

2
2
22




@ids_array


1,2


SET @ids_array := NULL;
INSERT INTO test (category, value)
VALUES (1,111), (2,22)
ON DUPLICATE KEY 
UPDATE value = NULL;
SELECT * FROM test;
SELECT @ids_array;


id
category
value


1
1
11

3
1
111

2
2
null




@ids_array


3,2


-- do not reset @ids_array
INSERT INTO test (id, category, value)
VALUES (1,4,44), (22,2,22)
ON DUPLICATE KEY 
UPDATE value = NULL;
SELECT * FROM test;
SELECT @ids_array;


id
category
value


1
1
null

3
1
111

2
2
null

22
2
22




@ids_array


3,2,1,22