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;
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:
Error Code : 1025
Error on rename of './data/#sql-4013_70f5e' to './data/parent' (errno: 150)
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?
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.
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)
I'll weigh in on this with what may be an unpopular suggestion. Don't use foreign key constraints in your database - enforce unique key and other constraints via TSQL in stored procedures as needed. It's my experience that in scaled environments check constraints are rarely used.
I say this with an open mind to opposing comments/discussion that may ensue. I'm not saying this suggestion is correct, just that it has been the prevailing opinion in the shops I've worked in.
A Request: If you downvote me, please also leave a short comment as well. In the 10 or so years I've been working with relational databases, the only people I know who use check constraints are working on systems that aren't at scale. If those are the people downvoting me then I can live with that. But if you're working on a scaled system and check constraints are the norm for you I'd like to know who you are so I can do some reading to see what I've missed.
来源:https://stackoverflow.com/questions/1191023/changing-mysql-primary-key-when-foreign-key-contraints-exist