add forign key in mysql alter table

CREATE TABLE IF NOT EXISTS `parent` (
  `id` int(11) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;


CREATE TABLE IF NOT EXISTS `child` (
  `id` int(11) DEFAULT NULL,
  `parent_id` int(11) DEFAULT NULL,
  KEY `par_ind` (`parent_id`),
  KEY `child_to_module_fk_company` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

ALTER TABLE `child`
  ADD CONSTRAINT `company_to_module_fk_company` 
    FOREIGN KEY (`parent_id`)
    REFERENCES `parent` (`id`)
    ON DELETE RESTRICT
    ON UPDATE RESTRICT;