如果我在InnoDB引擎上设计数据库，并且有3个表不能删除，因为每个表都说“外键约束失败”-这是否表示设计错误？

请参阅以下结构：

CREATE TABLE IF NOT EXISTS `account` (
  `account_id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`account_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;


CREATE TABLE IF NOT EXISTS `identity` (
  `identity_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `user_id` int(10) unsigned DEFAULT NULL,
  `account_id` smallint(5) unsigned DEFAULT NULL,
  PRIMARY KEY (`identity_id`),
  KEY `fk_details1` (`user_id`),
  KEY `fk_account1` (`account_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;


CREATE TABLE IF NOT EXISTS `user` (
  `user_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `login` varchar(64) NOT NULL DEFAULT '',
  `password` varchar(32) NOT NULL DEFAULT '',
  `default_identity_id` int(10) unsigned DEFAULT NULL,
  PRIMARY KEY (`user_id`),
  UNIQUE KEY `login_UNIQUE` (`login`),
  KEY `fk_identity1` (`default_identity_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

--
-- Constraints for table `identity`
--
ALTER TABLE `identity`
  ADD CONSTRAINT `fk_details1` FOREIGN KEY (`user_id`) REFERENCES `user` (`user_id`) ON DELETE CASCADE ON UPDATE CASCADE,
  ADD CONSTRAINT `fk_account1` FOREIGN KEY (`account_id`) REFERENCES `account` (`account_id`) ON DELETE CASCADE ON UPDATE CASCADE;

--
-- Constraints for table `user`
--
ALTER TABLE `user`
  ADD CONSTRAINT `fk_identity1` FOREIGN KEY (`default_identity_id`) REFERENCES `identity` (`identity_id`) ON DELETE CASCADE ON UPDATE CASCADE;

我怀疑问题出在default_identity_id …我应该将其作为标志移至身份表吗？

请指教！