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mysql> create table test (id int auto_increment primary key, val int);
Query OK, 0 rows affected (0.06 sec)

mysql> create trigger tr before insert on test for each row set new.val = new.id*new.id;
Query OK, 0 rows affected (0.05 sec)

mysql> insert into test values (10,10);
Query OK, 1 row affected (0.00 sec)

mysql> SET @TRIGGER_BEFORE_INSERT_CHECKS = FALSE;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into test values (20,20);
Query OK, 1 row affected (0.00 sec)

mysql> SET @TRIGGER_BEFORE_INSERT_CHECKS = TRUE;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into test values (30,30);
Query OK, 1 row affected (0.00 sec)

mysql> select * from test;
+----+------+
| id | val  |
+----+------+
| 10 |  100 |
| 20 |  400 |
| 30 |  900 |
+----+------+
3 rows in set (0.02 sec)

mysql>