UPDATE vn.town t
		LEFT JOIN vn.zoneGeo zg ON zg.id = t.geoFk
	SET t.geoFk = NULL
	WHERE zg.id IS NULL;

ALTER TABLE vn.town
	ADD CONSTRAINT town_zoneGeo_FK FOREIGN KEY (geoFk)
		REFERENCES vn.zoneGeo(id)
		ON DELETE RESTRICT
		ON UPDATE CASCADE;