MySQL foreign key constraint not applying? - mysql

Question

I have the tables Players and PlayerMeta
mysql> DESCRIBE Players;
+-------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+-------------+------+-----+---------+----------------+
| ID | int(5) | NO | PRI | NULL | auto_increment |
| PlayerName | varchar(20) | NO | PRI | NULL | |
| Birthdate | date | YES | | NULL | |
| Location | varchar(20) | YES | | NULL | |
| FirstName | varchar(15) | YES | | NULL | |
| Whitelisted | tinyint(1) | NO | | 1 | |
+-------------+-------------+------+-----+---------+----------------+
mysql> DESCRIBE PlayerMeta;
+----------------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------+------------+------+-----+---------+-------+
| ID | int(5) | NO | PRI | NULL | |
| JoinDate | date | YES | | NULL | |
| BuildQuota | int(2) | NO | | 2 | |
| RegisteredDate | date | YES | | NULL | |
| HideBirthdate | tinyint(1) | NO | | 0 | |
+----------------+------------+------+-----+---------+-------+
I am trying to execute this command, and it returns Query OK:
ALTER TABLE PlayerMeta
ADD CONSTRAINT fk_PlayerID
FOREIGN KEY (ID)
REFERENCES Players(ID)
ON UPDATE CASCADE
ON DELETE CASCADE;
Yet, when I run SHOW CREATE TABLE PlayerMeta, it does not show the constraint, nor is it in INFORMATION_SCHEMA
Any thoughts? Thanks.
EDIT: Here is SHOW CREATE TABLE PlayerMeta:
mysql> SHOW CREATE TABLE PlayerMeta;
... a bunch of lines ...
| PlayerMeta | CREATE TABLE `PlayerMeta` (
`ID` int(5) NOT NULL,
`JoinDate` date DEFAULT NULL,
`BuildQuota` int(2) NOT NULL DEFAULT '2',
`RegisteredDate` date DEFAULT NULL,
`HideBirthdate` tinyint(1) NOT NULL DEFAULT '0',
UNIQUE KEY `ID` (`ID`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 |
EDIT(2): The problem was ID in PlayerMeta was already a primary key and a foreign key would not apply in conjunction with it.

Answer 1

You cannot create foreign keys for MyISAM.
Only InnoDB supports them.
So the solution for you is to change the storage engine for both tables.
ALTER TABLE Players ENGINE=InnoDB;
ALTER TABLE PlayerMeta ENGINE=InnoDB;
Then re-apply your ALTER with adding a FK constraint.

Answer 2

MySQL will not permit you to create a FOREIGN KEY constraint on a column which is itself a PRIMARY KEY. If your design is such that the two tables share a one-to-one relationship and both hold information about a Player, it may not make sense to separate them and instead combine both tables into one.
In any case, if you wish to keep them separate and enforce a FOREIGN KEY relationship on them, you need to remove the PRIMARY KEY on PlayerMeta.ID. You may still have a UNIQUE, non primary key index on it.