Got an odd problem I cant solve after browsing dozens of forum posts, and my local SQL Books.
I've got two tables, and want to add a foreign key to one of them. The foreign key and primary key share the same datatype and charset and yet I cannot add the Foreign Key at all.
addon_account
| name | type | comments |
|---|---|---|
| id | int(11) | Primary Key |
| name | varchar(60) | Primary Key |
| label | varchar(255) | |
| shared | int(11) |
addon_account_data
| name | type | comments |
|---|---|---|
| id | int(11) | Primary Key |
| account_name | varchar(60) | Primary Key |
| money | double | |
| owner | varchar() |
The query I ran:
ALTER TABLE `addon_account_data` ADD FOREIGN KEY (`account_name`) REFERENCES `addon_account`(`name`) ON DELETE RESTRICT ON UPDATE RESTRICT;
Can't get it to work. Tosses out the same issue the entire time.
You are creating a foreign key on addon_account_data(account_name) that references addon_account(name). You have a composite primary the referred table : addon_account(id, name).
This is not allowed in MySQL, as explained in the documentation:
MySQL requires indexes on foreign keys and referenced keys so that foreign key checks can be fast and not require a table scan. In the referencing table, there must be an index where the foreign key columns are listed as the first columns in the same order.
Possible solutions:
add an additional column in the referring table: addon_account_data(account_id, account_name) and create a composite primary key to the corresponding columns in addon_account
create an index on addon_account(name) (probably the simplest solution)
change the order of the columns in the primary key of the referred table, like: addon_account(name, id) (you might want to first consider the impacts this may have in terms of performance)