问题描述
我有两个已经存在的表，看起来(部分)大致如下:
I have two already-existing tables which look (in part) roughly like this:
CREATE TABLE parent (
old_pk CHAR(8) NOT NULL PRIMARY KEY
) ENGINE=InnoDB;
CREATE TABLE child (
parent_key CHAR(8),
FOREIGN KEY (parent_key) REFERENCES parent(old_pk)
ON UPDATE CASCADE ON DELETE CASCADE
) ENGINE=InnoDB;
我想向 parent 添加一个新的自动递增整数 id 列并将其用作主键，同时仍保留 old_pk 作为唯一键并允许 child 等其他表在外键约束中引用它.不幸的是，简单地说 ALTER TABLE parent DROP PRIMARY KEY 不起作用:
I want to add a new auto-incrementing integer id column to parent and use it as the primary key instead, while still keeping old_pk as a unique key and allowing other tables like child to reference it in foreign key contraints. Unfortunately, simply saying ALTER TABLE parent DROP PRIMARY KEY doesn't work:
错误代码:1025
将./data/#sql-4013_70f5e"重命名为./data/parent"时出错(错误号:150)
Error on rename of './data/#sql-4013_70f5e' to './data/parent' (errno: 150)
一些谷歌搜索表明这是由于来自 child 的现有外键引用.本质上，我需要一种方法来告诉 MySQL使用另一列作为主键，但不要忘记原始列的唯一键".除了从 child 中删除关键约束并在之后恢复它们之外，还有什么方法可以做到这一点?
Some googling suggests that this is due to the existing foreign key reference from child. In essence, I need a way to tell MySQL "use this other column as the primary key, but don't forget the unique-key-ness of the original one". Is there any way to accomplish this, other than just dropping the key constraints from child and reinstating them afterwards?
假设我必须就地更改表，而不是创建具有相同数据的副本并在以后交换它们.我在更改表格之前尝试过使用 SET FOREIGN_KEY_CHECKS = 0，但它似乎没有帮助.
Assume that I must alter the tables in place, rather than creating copies with the same data and swapping them in later. I've tried using SET FOREIGN_KEY_CHECKS = 0 before altering the table, but it does not seem to help.
推荐答案
在删除主键之前给 old_pk 添加一个索引(甚至可以是唯一的):
Add an index (it could even be UNIQUE) to old_pk before dropping the primary key:
mysql> CREATE TABLE parent (
-> old_pk CHAR(8) NOT NULL PRIMARY KEY
-> ) ENGINE=InnoDB;
Query OK, 0 rows affected (0.00 sec)
mysql> CREATE TABLE child (
-> parent_key CHAR(8),
-> FOREIGN KEY (parent_key) REFERENCES parent(old_pk)
-> ON UPDATE CASCADE ON DELETE CASCADE
-> ) ENGINE=InnoDB;
Query OK, 0 rows affected (0.00 sec)
mysql> INSERT INTO parent VALUES ('a');
Query OK, 1 row affected (0.01 sec)
mysql> CREATE INDEX old_pk_unique ON parent (old_pk);
Query OK, 1 row affected (0.01 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE parent DROP PRIMARY KEY;
Query OK, 1 row affected (0.01 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> INSERT INTO child VALUES ('a');
Query OK, 1 row affected (0.00 sec)
mysql> SHOW CREATE TABLE parent;
+--------+------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+--------+------------------------------------------------------------------------------------------------------------------------------+
| parent | CREATE TABLE `parent` (
`old_pk` char(8) NOT NULL,
KEY `old_pk_unique` (`old_pk`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+--------+------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> INSERT INTO child VALUES ('b');
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test/child`, CONSTRAINT `child_ibfk_1` FOREIGN KEY (`parent_key`) REFERENCES `parent` (`old_pk`) ON DELETE CASCADE ON UPDATE CASCADE)
mysql> INSERT INTO parent VALUES ('b');
Query OK, 1 row affected (0.00 sec)
mysql> INSERT INTO child VALUES ('b');
Query OK, 1 row affected (0.01 sec)
mysql> ALTER TABLE parent ADD id INT;
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> UPDATE parent SET id = 1 WHERE old_pk = 'a';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> UPDATE parent SET id = 2 WHERE old_pk = 'b';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> ALTER TABLE parent ADD PRIMARY KEY (id);
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> SHOW CREATE TABLE parent;
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| parent | CREATE TABLE `parent` (
`old_pk` char(8) NOT NULL,
`id` int(11) NOT NULL default '0',
PRIMARY KEY (`id`),
KEY `old_pk_unique` (`old_pk`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
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