Trigger in MySQL "BEFORE DELETE" - mysql

Question

Deleting a record gives me below error:
ERROR 1442: 1442: Can't update table 'categoria' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.
This is my trigger:
DROP trigger IF EXISTS tg_eliminarCategoria;
DELIMITER $$
CREATE TRIGGER tg_eliminarCategoria
BEFORE DELETE ON Categoria
FOR EACH ROW
BEGIN
IF NOT EXISTS (SELECT * FROM Categoria WHERE nombreCategoria = 'Sin Categoria') THEN
INSERT INTO Categoria (nombreCategoria) VALUES ('Sin Categoria');
END IF;
SET #idCategoria = (SELECT idCategoria FROM Categoria WHERE nombreCategoria = 'Sin Categoria');
UPDATE Contacto SET idCategoria=#idCategoria WHERE idCategoria=OLD.idCategoria;
END$$
DELIMITER ;
The trigger is that by deleting a record from the "Categoria" table, it checks if there is a "Sin Categoria" record and if it does not exist, it modifies all records in the "Contacto" table containing the record deleted by The new record.
Sorry for my English, I'm still learning.

Answer 1

I guess that you want to make sure there is at least one row in the table for 'Sin Categoria'.
Another way to do this is to raise a SIGNAL (like an exception) if someone tries to delete 'Sin Categoria'.
mysql> DROP trigger IF EXISTS tg_eliminarCategoria;
mysql> DELIMITER $$
mysql> CREATE TRIGGER tg_eliminarCategoria
BEFORE DELETE ON Categoria
FOR EACH ROW
BEGIN
IF OLD.nombreCategoria = 'Sin Categoria' THEN
SIGNAL SQLSTATE '45000'
SET MESSAGE_TEXT = 'No debe eliminar la categoría especial: Sin Categoria';
END IF;
END$$
mysql> DELIMITER ;
mysql> delete from Categoria;
ERROR 1644 (45000): No debe eliminar la categoría especial: Sin Categoria
However, I think I have a better recommendation. I am guessing you use this for foreign keys that must reference a category, like this:
CREATE TABLE Contacto (
idContacto INT AUTO_INCREMENT PRIMARY KEY,
idCategoria INT NOT NULL,
FOREIGN KEY (idCategoria) REFERENCES Categoria(idCategoria)
);
When a Contacto has no Categoria, you reference the 'Sin Categoria' row. And you want to make sure no one deletes that row in the Categoria table.
Instead, I recommend using NULL when a row in Contacto has no Categoria. Just make Contacto.idCategoria allow NULL:
CREATE TABLE Contacto (
idContacto INT AUTO_INCREMENT PRIMARY KEY,
idCategoria INT DEFAULT NULL,
FOREIGN KEY (idCategoria) REFERENCES Categoria(idCategoria)
);
INSERT INTO Contacto
SET idContacto = 1234,
idCategoria = NULL;
Then you don't need any row in Categoria named "Sin Categoria".