Question

I want to add a second foreign key constraint between data and containers. When I delete a container, the data linked to this container has to be deleted too.

The tables:

mysql> DESCRIBE data;
+------------------------+--------------+------+-----+---------------------+----------------+
| Field                  | Type         | Null | Key | Default             | Extra          |
+------------------------+--------------+------+-----+---------------------+----------------
| imei                   | varchar(15)  | NO   | MUL | NULL                |                

mysql> DESCRIBE containers;
+--------------------+-------------+------+-----+---------------------+----------------+
| Field              | Type        | Null | Key | Default             | Extra          |
+--------------------+-------------+------+-----+---------------------+----------------+

| imei               | varchar(15) | NO   | MUL | NULL                |                |

I create a foreign key with this statement:

 mysql> ALTER TABLE `data` ADD FOREIGN KEY (`imei`) REFERENCES `containers`(`imei`) ON DELETE CASCADE;
    Query OK, 15168 rows affected (0.12 sec)
    Records: 15168  Duplicates: 0  Warnings: 0

But the foreign key hasn’t been created:

 mysql> use INFORMATION_SCHEMA;
    Reading table information for completion of table and column names
    You can turn off this feature to get a quicker startup with -A

    Database changed
    mysql> select TABLE_NAME,COLUMN_NAME,CONSTRAINT_NAME, REFERENCED_TABLE_NAME,REFERENCED_COLUMN_NAME from KEY_COLUMN_USAGE where REFERENCED_TABLE_NAME = 'containers';
    +----------------+-------------+-----------------------+-----------------------+------------------------+
    | TABLE_NAME     | COLUMN_NAME | CONSTRAINT_NAME       | REFERENCED_TABLE_NAME | REFERENCED_COLUMN_NAME |
    +----------------+-------------+-----------------------+-----------------------+------------------------+
    | container_logs | imei        | container_logs_ibfk_1 | containers            | imei                   |
    +----------------+-------------+-----------------------+-----------------------+------------------------+
    1 row in set (0.16 sec)

What am I doing wrong?

Answer 1

Not all storage engines support foreign keys; storage engines that does not support certain SQL features however do not generate error, but just ignore the statements (actually the SQL parser is higher layer in the MySQL architecture, which uses lower-level APIs to communicate with the storage engines).

So if you are using MyISAM, you cannot create a foreign key, but no error will be returned