Mysql 错误 1452 - 无法添加或更新子行：外键约束失败

我遇到了一个奇怪的问题。我正在尝试向引用另一个表的一个表添加外键，但由于某种原因失败了。由于我对 MySQL 的了解有限，唯一可能怀疑的是另一张表上有一个外键引用了我试图引用的表。

我做了一个SHOW CREATE TABLE对两个表进行查询，sourcecodes_tags是带有外键的表，sourcecodes是引用的表。

CREATE TABLE `sourcecodes` (
 `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
 `user_id` int(11) unsigned NOT NULL,
 `language_id` int(11) unsigned NOT NULL,
 `category_id` int(11) unsigned NOT NULL,
 `title` varchar(40) CHARACTER SET utf8 NOT NULL,
 `description` text CHARACTER SET utf8 NOT NULL,
 `views` int(11) unsigned NOT NULL,
 `downloads` int(11) unsigned NOT NULL,
 `time_posted` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
 PRIMARY KEY (`id`),
 KEY `user_id` (`user_id`),
 KEY `language_id` (`language_id`),
 KEY `category_id` (`category_id`),
 CONSTRAINT `sourcecodes_ibfk_3` FOREIGN KEY (`language_id`) REFERENCES `languages` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
 CONSTRAINT `sourcecodes_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
 CONSTRAINT `sourcecodes_ibfk_2` FOREIGN KEY (`category_id`) REFERENCES `categories` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=latin1

CREATE TABLE `sourcecodes_tags` (
 `sourcecode_id` int(11) unsigned NOT NULL,
 `tag_id` int(11) unsigned NOT NULL,
 KEY `sourcecode_id` (`sourcecode_id`),
 KEY `tag_id` (`tag_id`),
 CONSTRAINT `sourcecodes_tags_ibfk_1` FOREIGN KEY (`tag_id`) REFERENCES `tags` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=latin1

这是生成错误的代码：

ALTER TABLE sourcecodes_tags ADD FOREIGN KEY (sourcecode_id) REFERENCES sourcecodes (id) ON DELETE CASCADE ON UPDATE CASCADE

很可能你的sourcecodes_tags表包含sourcecode_id你的价值观中不再存在sourcecodes桌子。你必须先摆脱那些。

下面是一个可以找到这些 ID 的查询：

SELECT DISTINCT sourcecode_id FROM 
   sourcecodes_tags tags LEFT JOIN sourcecodes sc ON tags.sourcecode_id=sc.id 
WHERE sc.id IS NULL;