MySQL multiple fields foreign key referencing to single field - mysql

Question

I have languages table which looks like that:
-id
-name
-iso
and in multiple tables I need to reference this iso field as foreign key. The problem is, I can't do it even if I give completely unique FK names. What is problem?
Here is how my Languages table contructed:
CREATE TABLE `Languages` (
`id` tinyint(3) unsigned NOT NULL AUTO_INCREMENT,
`iso` char(2) NOT NULL,
`name` varchar(255) NOT NULL,
`description` varchar(255) NOT NULL,
`order` tinyint(3) NOT NULL DEFAULT '0',
`active` tinyint(1) NOT NULL DEFAULT '1',
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

Answer 1

Ensure that you have the same column datatype in your parent and child tables. Also create an index on your parent table's iso column. In absence of details in your question about tables you currently have and SQL you used for them, let's take some examples that you can compare with your database.
Example of different datatype:
create table languages (
id int,
name varchar(100),
iso char(3),
primary key(id)
);
create index ix_countries_iso on languages(iso);
create table countries (
id int,
iso int,
foreign key(iso) references languages(iso)
);
ERROR 1215 (HY000): Cannot add foreign key constraint
Example of missing index on parent table:
create table languages (
id int,
name varchar(100),
iso char(3),
primary key(id)
);
-- notice that we aren't creating an index on iso
create table countries (
id int,
iso char(3), -- notice correct datatype in child and parent table
foreign key(iso) references languages(iso)
);
ERROR 1215 (HY000): Cannot add foreign key constraint
What if you have the same datatype in parent and child but lengths are different? I'd recommend keeping the datatype and the length same but the following statements will work:
create table languages (
id int,
name varchar(100),
iso char(3),
primary key(id)
);
create index ix_countries_iso on languages(iso);
create table countries (
id int,
iso char(10), -- will work, but don't do this
foreign key(iso) references languages(iso)
);
create table countries2 (
id int,
iso char(1), -- will work also, but don't do this
foreign key(iso) references languages(iso)
);
Let's take an example of a working solution of foreign keys that you can compare with your database structure:
create table languages (
id int,
name varchar(100),
iso char(3),
primary key(id)
);
create index ix_countries_iso on languages(iso);
create table countries (
id int,
iso char(3), -- same datatype
foreign key fq_countries_languages(iso) references languages(iso)
);
create table boundaries (
id int,
iso char(3), -- same datatype
foreign key fq_boundaries_languages(iso) references languages(iso)
);
I'd also recommend that your child tables that rely on foreign key should be indexed also. Check out a nice little article on foreign keys.