DELIMITER $$
CREATE TRIGGER before_insert
BEFORE INSERT ON reservatie
FOR EACH ROW
BEGIN
DECLARE nmbr INT DEFAULT 0;
SELECT max_aantal_gasten
FROM accomodatie
WHERE accomodatie.accomodatiecode = new.accomodatiecode INTO nmbr;

IF (new.aantalgasten > nmbr)
THEN
SIGNAL SQLSTATE '45000'
SET MESSAGE_TEXT = 'ERROR te veel mensen',
TABLE_NAME = 'reservatie',
COLUMN_NAME='aantalgasten',
MYSQL_ERRNO = 9005;
END IF;

END $$
DELIMITER :