可以创建多个连接表的路径吗？

问题描述

假设我有一个用户表：

CREATE TABLE `users` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(255) NOT NULL,
  `city_id` bigint(20) unsigned NOT NULL,
  PRIMARY KEY (`id`),
  KEY `users_city_id_foreign` (`city_id`),
  CONSTRAINT `users_city_id_foreign` FOREIGN KEY (`city_id`) REFERENCES `cities` (`id`)
);

这链接到一个城市表：

CREATE TABLE `cities` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(255) NOT NULL,
  `country_id` bigint(20) unsigned NOT NULL,
  PRIMARY KEY (`id`),
  KEY `cities_country_id_foreign` (`country_id`),
  CONSTRAINT `cities_country_id_foreign` FOREIGN KEY (`country_id`) REFERENCES `countries` (`id`)
);

这与国家表相关联：

CREATE TABLE `countries` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(255) NOT NULL,
  PRIMARY KEY (`id`)
);

我经常需要用户的国家名称。 我已经可以通过user->city->country访问国家名称：

SELECT `users`.`name`, `countries`.`name`
FROM `users` JOIN 
    (`cities` JOIN `countries` ON `cities`.`country_id` = `countries`.`id`)
    ON `users`.`city_id` = `cities`.`id`;

但我想直接访问，例如：

SELECT `users`.`name`, `countries`.`name`
FROM `users` JOIN `countries` ON `users`.`country_id` = `countries`.`id

可以像这样添加一个将用户表直接链接到国家表的外键，还是应该避免它？

ALTER TABLE `users` 
ADD COLUMN `country_id` BIGINT(20) UNSIGNED NOT NULL AFTER `city_id`,
ADD INDEX `users_country_id_foreign_idx` (`country_id` ASC);

ALTER TABLE `users` 
ADD CONSTRAINT `users_country_id_foreign`
  FOREIGN KEY (`country_id`)
  REFERENCES `countries` (`id`)
  ON DELETE NO ACTION
  ON UPDATE NO ACTION;