如何在 Mysql 中创建复合外键

Question

我需要将复合外键添加到 table 结构看起来像

CREATE TABLE IF NOT EXISTS `discount_month_devices` (
  `id` int(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  `discount_month_id` int(11) UNSIGNED NOT NULL,
  `global_device_id` int(11) DEFAULT NULL,
  `location_id` int(11) UNSIGNED DEFAULT NULL,
  `server_id` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `discount_month_id` (`discount_month_id`),
  KEY `global_device_id` (`global_device_id`),
  KEY `location_id` (`location_id`,`server_id`),
  KEY `server_id` (`server_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

设备table DDL 看起来像

CREATE TABLE IF NOT EXISTS `devices` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `server_id` varchar(20) CHARACTER SET utf8mb4 NOT NULL,
  `device_id` int(11) DEFAULT NULL,
  `location_id` int(11) UNSIGNED DEFAULT NULL,
  `device_lat` float DEFAULT NULL,
  `device_long` float DEFAULT NULL,
   ....
  PRIMARY KEY (`id`) USING BTREE,
  UNIQUE KEY `devices_idx1` (`server_id`,`device_id`) USING BTREE,
  KEY `devices_idx5` (`server_id`) USING BTREE,
  KEY `devices_idx6` (`device_id`) USING BTREE,
  KEY `devices_idx8` (`server_id`,`owner_id`) USING BTREE,
  KEY `server_id` (`server_id`,`location_id`),
  KEY `devices_idx14` (`location_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1583586 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci ROW_FORMAT=DYNAMIC;

ALTER TABLE `devices`
  ADD CONSTRAINT `devices_fk1` FOREIGN KEY (`server_id`,`location_id`) REFERENCES `locations` (`server_id`, `location_id`) ON DELETE CASCADE ON UPDATE CASCADE,
  ADD CONSTRAINT `devices_fk2` FOREIGN KEY (`discount_month_id`) REFERENCES `discount_month` (`id`);

有location_id个复合索引。我可以分别为 location_id 和 server_id 创建 FK，因此列类型和范围应该是正确的。

我想运行改变table应该添加看起来像

的外国

ALTER TABLE `discount_month_devices` ADD CONSTRAINT `discount_month_devices_fk3` 
FOREIGN KEY (`location_id`, `server_id`) REFERENCES `devices`(`location_id`, `server_id`) 
ON DELETE CASCADE ON UPDATE CASCADE;

这会抛出一个错误：一般错误：1215 无法添加外键约束

有谁知道可能是什么问题。

Answer 1

您必须按照它们在引用的 table 的键中出现的相同顺序列出外键约束中的列。您在 devices 中的键在 (server_id, location_id) 上，但您试图在外键约束中将它们引用为 (location_id, server_id)。

试试这个：

ALTER TABLE `discount_month_devices` 
  ADD CONSTRAINT `discount_month_devices_fk3` 
  FOREIGN KEY (`server_id`, `location_id`) 
  REFERENCES `devices`(`server_id`, `location_id`)
  ON DELETE CASCADE ON UPDATE CASCADE;

键和约束中的列顺序不需要与 table 定义中的列顺序相匹配。

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