Problem adding Foreign Key using Alter Table with existing MYSQL Database - can't add it! Help!

Question

I have a production database where I have renamed several column\'s that are foreign keys. Obviously mysql makes this a real pain to do in my experience.

My solution

Answer 1

Are both tables InnoDB type?

Does the company table have an index on company_id ?

I guess that your table is MyISAM (the default if you haven't changed the config) and you can't create foreign key constraints in MyISAM. See the description of the CREATE TABLE for yout two tables.

If both tables are empty, drop them and re-create them, choosing InnoDB as engine. You could also add the FOREIGN KEY constraints in the tables creation script(s).

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From MySQL Reference Manual:

Foreign keys definitions are subject to the following conditions:

• Both tables must be InnoDB tables and they must not be TEMPORARY tables.

• Corresponding columns in the foreign key and the referenced key must have similar internal data types inside InnoDB so that they can be compared without a type conversion. The size and sign of integer types must be the same. The length of string types need not be the same. For nonbinary (character) string columns, the character set and collation must be the same.

• InnoDB 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. Such an index is created on the referencing table automatically if it does not exist. (This is in contrast to some older versions, in which indexes had to be created explicitly or the creation of foreign key constraints would fail.) index_name, if given, is used as described previously.

• InnoDB permits a foreign key to reference any index column or group of columns. However, in the referenced table, there must be an index where the referenced columns are listed as the first columns in the same order.

• Index prefixes on foreign key columns are not supported. One consequence of this is that BLOB and TEXT columns cannot be included in a foreign key because indexes on those columns must always include a prefix length.

• If the CONSTRAINT symbol clause is given, the symbol value must be unique in the database. If the clause is not given, InnoDB creates the name automatically.

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@egervari: What happens if you run this:

CREATE TABLE `test` (
  `company_id` bigint(20) NOT NULL,
  `module_id` bigint(20) NOT NULL,

  KEY  (`module_id`),
  KEY  (`company_id`),

  CONSTRAINT `test_fk_module`
    FOREIGN KEY (`module_id`)
    REFERENCES `module` (`module_id`),

  CONSTRAINT `test_fk_company`
    FOREIGN KEY (`company_id`)
    REFERENCES `company` (`company_id`)
    ON DELETE RESTRICT
    ON UPDATE RESTRICT

) ENGINE=InnoDB DEFAULT CHARSET=utf8 ;

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And if you run:

ALTER TABLE `company_to_module`
  ADD CONSTRAINT `company_to_module_fk_company` 
    FOREIGN KEY (`company_id`)
    REFERENCES `company` (`company_id`)
    ON DELETE RESTRICT
    ON UPDATE RESTRICT;