Throw an error in a MySQL trigger

Question

If I have a trigger before the update on a table, how can I throw an error that prevents the update on that table?

Answer 1

Unfortunately, the answer provided by @RuiDC does not work in MySQL versions prior to 5.5 because there is no implementation of SIGNAL for stored procedures.

The solution I've found is to simulate a signal throwing a table_name doesn't exist error, pushing a customized error message into the table_name.

The hack could be implemented using triggers or using a stored procedure. I describe both options below following the example used by @RuiDC.

Using triggers

DELIMITER $$
-- before inserting new id
DROP TRIGGER IF EXISTS before_insert_id$$
CREATE TRIGGER before_insert_id
    BEFORE INSERT ON test FOR EACH ROW
    BEGIN
        -- condition to check
        IF NEW.id < 0 THEN
            -- hack to solve absence of SIGNAL/prepared statements in triggers
            UPDATE `Error: invalid_id_test` SET x=1;
        END IF;
    END$$

DELIMITER ;

Using a stored procedure

Stored procedures allows you to use dynamic sql, which makes possible the encapsulation of the error generation functionality in one procedure. The counterpoint is that we should control the applications insert/update methods, so they use only our stored procedure (not granting direct privileges to INSERT/UPDATE).

DELIMITER $$
-- my_signal procedure
CREATE PROCEDURE `my_signal`(in_errortext VARCHAR(255))
BEGIN
    SET @sql=CONCAT('UPDATE `', in_errortext, '` SET x=1');
    PREPARE my_signal_stmt FROM @sql;
    EXECUTE my_signal_stmt;
    DEALLOCATE PREPARE my_signal_stmt;
END$$

CREATE PROCEDURE insert_test(p_id INT)
BEGIN
    IF NEW.id < 0 THEN
         CALL my_signal('Error: invalid_id_test; Id must be a positive integer');
    ELSE
        INSERT INTO test (id) VALUES (p_id);
    END IF;
END$$
DELIMITER ;