Unable to use SIGNAL and INSERT at the same time - mysql

Question

I have a problem with this trigger. When I use the INSERT and the SIGNAL statements inside the IF condition, the insertion is not performed. However, without SIGNAL, the insertion is done. Anyone have an explanation for that? My main concern is that I need both the insertion and the SIGNAL statement to cancel the main insertion (the insertion that throws the trigger)
DELIMITER //
CREATE TRIGGER log_venta BEFORE INSERT ON `venta_producto`
FOR EACH ROW BEGIN
DECLARE value int;
DECLARE valor_venta int;
DECLARE saldo_cliente int;
DECLARE cliente_id int;
SELECT cliente INTO cliente_id FROM venta WHERE id=NEW.ventaID;
SELECT credito INTO saldo_cliente FROM cliente WHERE no_cliente=cliente_id;
SELECT precio INTO valor_producto FROM producto WHERE id=NEW.producto;
SET valor_venta = NEW.cantidad*valor_producto;
IF valor_venta > saldo_cliente THEN
INSERT INTO log(cliente) VALUES (cliente_id);
SIGNAL SQLSTATE '02000' SET MESSAGE_TEXT = 'ERROR';
END IF;
END
//
DELIMITER ;
Thanks

Answer 1

14.6.7.5 SIGNAL Syntax
. . .
SIGNAL is the
way to “return” an error.
. . .
21.3.1 Trigger Syntax and Examples
. . .
For transactional tables, failure of a statement should cause rollback of all changes performed by the statement. Failure of a
trigger causes the statement to fail, so trigger failure also causes
rollback. For nontransactional tables, such rollback cannot be done,
so although the statement fails, any changes performed prior to the
point of the error remain in effect.
. . .
Mentioned above can be demonstrated in the following example:
mysql> DROP TABLE IF EXISTS `venta_producto`;
Query OK, 0 rows affected (0.00 sec)
mysql> DROP TABLE IF EXISTS `log`;
Query OK, 0 rows affected (0.00 sec)
mysql> CREATE TABLE IF NOT EXISTS `venta_producto` (
-> `id` INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
-> `cliente_id` INT UNSIGNED
-> );
Query OK, 0 rows affected (0.01 sec)
mysql> CREATE TABLE IF NOT EXISTS `log` (
-> `cliente_id` INT UNSIGNED
-> ) ENGINE=InnoDB;
Query OK, 0 rows affected (0.00 sec)
mysql> DELIMITER //
mysql> CREATE TRIGGER `log_venta` BEFORE INSERT ON `venta_producto`
-> FOR EACH ROW
-> BEGIN
-> INSERT INTO `log` (`cliente_id`) VALUES (NEW.`cliente_id`);
-> SIGNAL SQLSTATE '02000' SET MESSAGE_TEXT = 'ERROR';
-> END//
Query OK, 0 rows affected (0.00 sec)
mysql> DELIMITER ;
mysql> INSERT INTO `venta_producto`
-> (`cliente_id`)
-> VALUES
-> (1);
ERROR 1643 (02000): ERROR
mysql> SELECT
-> `id`,
-> `cliente_id`
-> FROM
-> `venta_producto`;
Empty set (0.00 sec)
mysql> SELECT
-> `cliente_id`
-> FROM
-> `log`;
Empty set (0.00 sec)
mysql> ALTER TABLE `log` ENGINE=MyISAM;
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> INSERT INTO `venta_producto`
-> (`cliente_id`)
-> VALUES
-> (2);
ERROR 1643 (02000): ERROR
mysql> SELECT
-> `id`,
-> `cliente_id`
-> FROM
-> `venta_producto`;
Empty set (0.00 sec)
mysql> SELECT
-> `cliente_id`
-> FROM
-> `log`;
+------------+
| cliente_id |
+------------+
| 2 |
+------------+
1 row in set (0.00 sec)