Mysql, in the unique index of combination, handles the problem of null value

I have a problem recently , Use the following test table to describe ,

create table TEST
(
  col_1   INT,
  col_2   INT,
  col_3   VARCHAR(128) not null,
  col_4   VARCHAR(128) not null,
  UNIQUE KEY (col_1, col_2, col_3),
  CONSTRAINT FOREIGN KEY (col_1) REFERENCES T2(ID) ON DELETE CASCADE,
  CONSTRAINT FOREIGN KEY (col_2) REFERENCES T3(ID) ON DELETE CASCADE

)
  comment=' The test table '
  ENGINE=InnoDB DEFAULT CHARACTER SET=utf8;

The original intention is to require the following ：

INSERT INTO TEST (col_1, col_2, col_3, col_4) VALUES (1, NULL, 'TEST', 'VALID'); //OK
INSERT INTO TEST (col_1, col_2, col_3, col_4) VALUES (1, NULL, 'TEST', 'VALID'); //ERROR

But actually MYSQL But you can insert data , No hint of violation of UNIQUE KEY (col_1, col_2, col_3), This is MYSQL Yes NULL Caused by the method of treatment . Although many users are giving MYSQL Mention the N About this BUG, however MYSQL Dead hug SQL specs Don't put , Just don't change or provide alternative solutions .

Is there any way to satisfy my original intention ？ Someone has provided such a method ：

create table TEST
(
  ID      INT not null AUTO_INCREMENT,
  col_1   INT,
  col_2   INT,
  col_3   VARCHAR(128) not null,
  col_4   VARCHAR(128) not null,
  PRIMARY KEY (ID),
  CONSTRAINT FOREIGN KEY (col_1) REFERENCES T2(ID) ON DELETE CASCADE,
  CONSTRAINT FOREIGN KEY (col_2) REFERENCES T3(ID) ON DELETE CASCADE
)
  comment=' The test table '
  ENGINE=InnoDB DEFAULT CHARACTER SET=utf8;

First , Remove the unique index of the combination , Instead, it adds a self growth PK：ID
secondly , Create a for this table triigger:

CREATE TRIGGER `TEST_BEFORE_INS` 
BEFORE INSERT ON `TEST` FOR EACH ROW
BEGIN
SET NEW.id=(SELECT id FROM TEST WHERE col_1<=>NEW.col_1 AND col_2<=>NEW.col_2 AND col_3<=>NEW.col_3);
END

Let's test it again ：

INSERT INTO TEST (col_1, col_2, col_3, col_4) VALUES (1, NULL, 'TEST', 'VALID'); //OK
INSERT INTO TEST (col_1, col_2, col_3, col_4) VALUES (1, NULL, 'TEST', 'VALID'); //ERROR：Duplicate entry '1' for key 'PRIMARY'

Report duplicate primary key error , It seems , It basically meets my original needs .