非唯一键的 Mysql 外键——这怎么可能？

Question

我在将 mysql 数据库迁移到 postgres 时无意中发现了 DDL 中的以下 block (注意:这是我从 mysqldump 得到的):

CREATE TABLE `catalog_property_value` (
  `id` int(10) unsigned NOT NULL,
  `property_id` int(10) unsigned NOT NULL,
  `sort` int(10) unsigned NOT NULL,
  `value_number` decimal(15,5) DEFAULT NULL,
  `value_string` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`,`sort`),
  KEY `FK_catalog_property_value` (`property_id`),
  KEY `NewIndex1` (`id`),
  CONSTRAINT `FK_catalog_property_value` FOREIGN KEY (`property_id`) REFERENCES `catalog_property` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
SET character_set_client = @saved_cs_client;

CREATE TABLE `catalog_realty_property_value_link` (
  `realty_id` int(10) unsigned NOT NULL,
  `property_id` int(10) unsigned NOT NULL,
  `value_id` int(10) unsigned NOT NULL,
  `dt_is_denormalized` tinyint(1) unsigned NOT NULL,
  PRIMARY KEY (`realty_id`,`property_id`,`value_id`),
  KEY `FK_catalog_realty_property_value_link_property` (`property_id`),
  KEY `FK_catalog_realty_property_value_link_value` (`value_id`),
  CONSTRAINT `FK_catalog_realty_property_value_link_property` FOREIGN KEY (`property_id`) REFERENCES `catalog_property` (`id`) ON DELETE CASCADE,
  CONSTRAINT `FK_catalog_realty_property_value_link_realty` FOREIGN KEY (`realty_id`) REFERENCES `catalog_realty` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `FK_catalog_realty_property_value_link_value` FOREIGN KEY (`value_id`) REFERENCES `catalog_property_value` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

现在，我在这里看到的是第一个表中唯一的唯一键是 (id, sort) 的组合:

PRIMARY KEY (`id`,`sort`),

但是，第二个表仅通过 id 列引用了第一个表，这不是唯一的!

CONSTRAINT `FK_catalog_realty_property_value_link_value` FOREIGN KEY (`value_id`) REFERENCES `catalog_property_value` (`id`) ON DELETE CASCADE

那么，我在这里做错了什么？这怎么可能？

Answer 1

来自手册:

Deviation from SQL standards: A FOREIGN KEY constraint that references a non-UNIQUE key is not standard SQL. It is an InnoDB extension to standard SQL.

所以看起来 InnoDB 允许非唯一索引作为外键引用的候选者。手册在其他地方指出，您可以在引用的索引中引用列的子集，只要引用的列首先列出并且顺序与主键相同。

因此，这个定义在 InnoDB 中是合法的，尽管它不是标准的 SQL，至少让我对最初设计者的意图有点困惑。

Manual page here .