Question

涉及的表格：

phppos_permissions_actions：

mysql> show create table phppos_permissions_actions;
+----------------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table                      | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
+----------------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| phppos_permissions_actions | CREATE TABLE `phppos_permissions_actions` (
  `module_id` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
  `person_id` int(11) NOT NULL,
  `action_id` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
  PRIMARY KEY (`module_id`,`person_id`,`action_id`),
  KEY `phppos_permissions_actions_ibfk_2` (`person_id`),
  KEY `phppos_permissions_actions_ibfk_3` (`action_id`),
  CONSTRAINT `phppos_permissions_actions_ibfk_1` FOREIGN KEY (`module_id`) REFERENCES `phppos_modules` (`module_id`),
  CONSTRAINT `phppos_permissions_actions_ibfk_2` FOREIGN KEY (`person_id`) REFERENCES `phppos_employees` (`person_id`),
  CONSTRAINT `phppos_permissions_actions_ibfk_3` FOREIGN KEY (`action_id`) REFERENCES `phppos_modules_actions` (`action_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci |
+----------------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

phppos_modules

mysql> show create table phppos_modules;
+----------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table          | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                       |
+----------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| phppos_modules | CREATE TABLE `phppos_modules` (
  `name_lang_key` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
  `desc_lang_key` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
  `sort` int(10) NOT NULL,
  `module_id` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
  PRIMARY KEY (`module_id`),
  UNIQUE KEY `desc_lang_key` (`desc_lang_key`),
  UNIQUE KEY `name_lang_key` (`name_lang_key`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci |
+----------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

查询：

ALTER TABLE `phppos_permissions_actions` CHANGE `module_id` `module_id` VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL, CHANGE `action_id` `action_id` VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL

错误：

#1832 - Cannot change column 'module_id': used in a foreign key constraint 'phppos_permissions_actions_ibfk_1'

（在mysql 5.5中没有错误）

我能让它发挥作用的唯一方法是：

SET foreign_key_checks = 0;

5.6中有什么变化？这是一个错误吗？

Answer 1

这显然是MySQL 5.6的改进，虽然实现看起来有点过于严格。

http://dev.mysql.com/doc/refman/5.6/en/alter-table.html说：

从5.6.7开始，服务器禁止更改外键列，否则可能导致参照完整性丢失。解决方法是在更改列定义之前使用ALTER TABLE ... DROP FOREIGN KEY，然后使用ALTER TABLE ... ADD FOREIGN KEY。

发行说明说这与http://bugs.mysql.com/bug.php?id=46599

有关

那很好，但是......

http://dev.mysql.com/doc/refman/5.6/en/create-table-foreign-keys.html说：

外键和引用键中的对应列必须具有相似的数据类型。整数类型的大小和符号必须相同。 字符串类型的长度不必相同。对于非二进制（字符）字符串列，字符集和排序规则必须相同。

重新评论：

这并不是一个矛盾。您仍然可以使用不同的字符串长度在MySQL 5.6中创建外键。

create table foo ( p varchar(20) primary key );
create table bar ( f varchar(10), foreign key (f) references foo (p) );

只要没有截断数据的可能性，您就可以修改列。

alter table bar modify column f varchar(20); /* increasing string length */
Query OK

但如果列可能会丢失数据，则无法对其进行修改。

alter table bar modify column f varchar(5); /* decreasing string length */
ERROR 1832 (HY000): Cannot change column 'f': 
used in a foreign key constraint 'bar_ibfk_1'

正如您所发现的，您可以使用foreign_check_checks=0禁用外键检查，或者通过使用ALTER TABLE删除约束，然后在修改列后重新创建约束。

mysql 5.6外键约束错误;没有发生在5.5

1 个答案: