Cannot ADD FOREIGN KEY using ALTER TABLE in MySQL

Question

I want to add a foreign key to column USERNAME in table data_access_tokens , and make it reference to the parent table users column EMAIL .

I have checked (1) both USERNAME (in the child table) and EMAIL (in the parent table) are VARCHAR(128) NOT NULL; (2) EMAIL in the parent table is primary key (3) both tables use default DB engine and codepages.

Why the following SQL command to add a foreign failed:

mysql> ALTER TABLE `data_access_tokens`
    ->    ADD FOREIGN KEY (`USERNAME`)
    ->    REFERENCES `users` (`EMAIL`)
    ->    ON DELETE CASCADE;
ERROR 1215 (HY000): Cannot add foreign key constraint

This is the parent table ( users ):

mysql> show create table users;
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                          |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| users | CREATE TABLE `users` (
  `EMAIL` varchar(128) NOT NULL,
  `NAME` varchar(255) NOT NULL,
  `ENABLED` tinyint(1) NOT NULL,
  PRIMARY KEY (`EMAIL`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> desc users;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| EMAIL   | varchar(128) | NO   | PRI | NULL    |       |
| NAME    | varchar(255) | NO   |     | NULL    |       |
| ENABLED | tinyint(1)   | NO   |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

This is the child table ( data_access_tokens ):

mysql> show create table data_access_tokens;
+--------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table              | Create Table                                                                                                                                                                                                                                 |
+--------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| data_access_tokens | CREATE TABLE `data_access_tokens` (
  `TOKEN` varchar(50) NOT NULL,
  `USERNAME` varchar(128) NOT NULL,
  `EXPIRATION` datetime NOT NULL,
  `CREATION` datetime DEFAULT NULL,
  PRIMARY KEY (`TOKEN`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+--------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> desc data_access_tokens;
+------------+--------------+------+-----+---------+-------+
| Field      | Type         | Null | Key | Default | Extra |
+------------+--------------+------+-----+---------+-------+
| TOKEN      | varchar(50)  | NO   | PRI | NULL    |       |
| USERNAME   | varchar(128) | NO   |     | NULL    |       |
| EXPIRATION | datetime     | NO   |     | NULL    |       |
| CREATION   | datetime     | YES  |     | NULL    |       |
+------------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

Answer 1

In this case, the problem is that the character sets for the two tables are different. Columns in data_access_tokens are latin1 , but the column it references in users is utf8 .

You can fix this:

ALTER TABLE data_access_tokens CONVERT TO CHARACTER SET utf8;

Like many ALTER TABLE statements, this will lock the table and rewrite it. It may take some time if the table is large.