MySQL foreign key constraints and triggers, cascading delete

-- Create test main table. ID is the primary key.
CREATE TABLE test_main (
  id INT NOT NULL,
  value VARCHAR(10),
  PRIMARY KEY(id)  
);
  
-- Create test sub table. 
CREATE TABLE test_sub (
  id INT NOT NULL,
  main_id INT ,
  value VARCHAR(10),
  PRIMARY KEY(id)  
); 
 
-- Insert test main table data.
INSERT INTO test_main(id, value) VALUES (1, 'ONE');
INSERT INTO test_main(id, value) VALUES ( 2, 'TWO');
 
-- Insert test subtable data.
INSERT INTO test_sub(id, main_id, value) VALUES (1, 1, 'ONEONE');
INSERT INTO test_sub(id, main_id, value) VALUES (2, 2, 'TWOTWO');

mysql> ALTER TABLE test_sub
    -> ADD CONSTRAINT main_id_cons -
    > FOREIGN KEY (main_id)
    -> REFERENCES test_main(id)
    -> ON DELETE CASCADE//
Query OK, 2 rows affected (0.16 sec)
Records: 2 Duplicates: 0 Warnings: 0
 
mysql> DELETE FROM
    -> test_main
    -> WHERE
    -> id = 1;
    -> //
Query OK, 1 row affected (0.02 sec)
 
mysql> SELECT
    -> *
    -> FROM
    -> test_sub;
    ->//
+----+---------+--------+
| id | main_id | value  |
+----+---------+--------+
|  2 |       2 | TWOTWO |
+----+---------+--------+
1 row in set (0.00 sec)

If you insist on using triggers, here is an example:

-- Create test main table. ID is the primary key.
CREATE TABLE t_test_main (
  id INT NOT NULL,
  value VARCHAR(10),
  PRIMARY KEY(id)  
);
  
-- Create test sub table. 
CREATE TABLE t_test_sub (
  id INT NOT NULL,
  main_id INT ,
  value VARCHAR(10),
  PRIMARY KEY(id)  
);
  
-- Insert test main table data.
INSERT INTO t_test_main(id, value) VALUES (1, 'ONE');
INSERT INTO t_test_main(id, value) VALUES ( 2, 'TWO');
 
-- Insert test subtable data.
INSERT INTO t_test_sub(id, main_id, value) VALUES (1, 1, 'ONEONE');
INSERT INTO t_test_sub(id, main_id, value) VALUES (2, 2, 'TWOTWO');
  
 
DELIMITER //
 
CREATE TRIGGER tr_t_test_main_Del
  BEFORE DELETE ON t_test_main
FOR EACH ROW
BEGIN
  DELETE FROM t_test_sub WHERE main_id = OLD.id;
END;
//
 
DELIMITER ;
 
  
mysql> delete from  t_test_main  where id = 1;
Query OK, 1 row affected (0.01 sec)
 
mysql> select * from t_test_sub;
+----+---------+--------+
| id | main_id | value  |
+----+---------+--------+
|  2 |       2 | TWOTWO |
+----+---------+--------+
1 row in set (0.00 sec)