MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

SELECT @@sql_mode;

@@sql_mode

ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION

CREATE TABLE test (
  id INT AUTO_INCREMENT PRIMARY KEY,
  val INT,
  gen INT AS (val + 123)
) AUTO_INCREMENT = 789;
CREATE TRIGGER tr
BEFORE INSERT ON test
FOR EACH ROW
SET @debug = CONCAT_WS('; ', COALESCE(NEW.id, -1), COALESCE(NEW.val, -2), COALESCE(NEW.gen, -3))
INSERT INTO test (val) SELECT 321;
SELECT * FROM test;
SELECT @debug;

Records: 1  Duplicates: 0  Warnings: 0

id
val
gen

789
321
444

@debug

0; 321; 444

INSERT INTO test (id, val) SELECT NULL, 456;
SELECT * FROM test;
SELECT @debug;

Records: 1  Duplicates: 0  Warnings: 0

id
val
gen

789
321
444

790
456
579

@debug

0; 456; 579