Can't create FOREIGN KEY CONSTRAINT in MySQL

Question

I created my MySQL database using phpMyAdmin 3.5.8.1deb1 in Ubuntu.

Instead of that all my tables are InnoDB, I can't add a foreign key, and this is an example:

ALTER TABLE element ADD CONSTRAINT FK_element_id_user FOREIGN KEY (id_user) REFERENCES user(id) ON DELETE SET NULL ON UPDATE CASCADE;

When I run this script I get this error :

#1005 - Can't create table 'tpw.#sql-4d8_e2' (errno: 150) (Details...)

When I click on details I get this :

InnoDB Documentation Supports transactions, row-level locking, and foreign keys

I tried to add the FK manually in the relation view

Solution

There could be a couple of things going one here. Here are some things to look for:

• Do the data types of each field between the tables match?
• Are both both tables using the same MySQL engine?

Here is a good resource to help you debug this issue further.

Excerpt from the resource linked to above:

1) The two key fields type and/or size is not an exact match. For example, if one is INT(10) the key field needs to be INT(10) as well and not INT(11) or TINYINT. You may want to confirm the field size using SHOW CREATE TABLE because Query Browser will sometimes visually show just INTEGER for both INT(10) and INT(11). You should also check that one is not SIGNED and the other is UNSIGNED. They both need to be exactly the same.

2) One of the key field that you are trying to reference does not have an index and/or is not a primary key. If one of the fields in the relationship is not a primary key, you must create an index for that field.

3) The foreign key name is a duplicate of an already existing key. Check that the name of your foreign key is unique within your database. Just add a few random characters to the end of your key name to test for this.

4) One or both of your tables is a MyISAM table. In order to use foreign keys, the tables must both be InnoDB. (Actually, if both tables are MyISAM then you won’t get an error message – it just won’t create the key.) In Query Browser, you can specify the table type.

5) You have specified a cascade ON DELETE SET NULL, but the relevant key field is set to NOT NULL. You can fix this by either changing your cascade or setting the field to allow NULL values.

6) Make sure that the Charset and Collate options are the same both at the table level as well as individual field level for the key columns.

7) You have a default value (ie default=0) on your foreign key column.

8) One of the fields in the relationship is part of a combination (composite) key and does not have it’s own individual index. Even though the field has an index as part of the composite key, you must create a separate index for only that key field in order to use it in a constraint.

9) You have a syntax error in your ALTER statement or you have mistyped one of the field names in the relationship.

10) The name of your foreign key exceeds the max length of 64 chars.

Comments

[Wouter Vanherck]

None of the above seem to be the issue in my case, however I'll take it that they're all correct answers. Could it be that my FK is failing because there's already another child with an FK pointed to the same parent? Also: the link's broke :(

[Stewii]

You might include whether some fields have "USING BTREE" while others do not (or another algorithm name). I just finished bashing my head against the keyboard for three hours to track that down. Thanks HeidiSQL ><