CREATE DEFINER = 'root'@'localhost' TRIGGER test.trigger1
	BEFORE INSERT 	ON test.test
	FOR EACH ROW BEGIN   set new.weight=new.id; END

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mysql> CREATE TABLE IF NOT EXISTS `test` (
    ->  `id` int(14)  auto_increment,
    ->  `parent` varchar(50) default '0',
    ->  `weight` int(14) NULL,
    ->  PRIMARY KEY  (`id`)
    -> );
Query OK, 0 rows affected (0.14 sec)

mysql> CREATE TRIGGER test.trigger1
    -> BEFORE INSERT ON test.test
    -> FOR EACH ROW set new.weight=new.id;
Query OK, 0 rows affected (0.20 sec)

mysql> INSERT INTO test(parent) VALUES (null),(1),(1),(null),(3);
Query OK, 5 rows affected (0.04 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM test;
+----+--------+--------+
| id | parent | weight |
+----+--------+--------+
|  1 | NULL   |      0 |
|  2 | 1      |      0 |
|  3 | 1      |      0 |
|  4 | NULL   |      0 |
|  5 | 3      |      0 |
+----+--------+--------+
5 rows in set (0.00 sec)