Causing MySQL to raise an exception from a trigger

gistfile1.sql

-- MySQL does not implement raising exceptions within triggers. Here's a hacky model for doing so.
-- This permits unwanted conditions to bubble up to the application layer, to be dealt with
-- Only use this during debugging. Don't rely on this in production code as it's a hack.

DELIMITER $$

-- Procedure guaranteed to raise an exception
CREATE PROCEDURE die_horribly() SQL SECURITY INVOKER DETERMINISTIC
BEGIN
  INSERT INTO does_not_exist (nid) VALUES (NULL);
END;

$$

-- Procedure guaranteed to raise an exception
CREATE TRIGGER call_die BEFORE UPDATE ON node_revisions
FOR EACH ROW
BEGIN
  IF NEW.status = 0 THEN CALL die_horribly();
  END IF;
END;
$$

DELIMITER ;