MySQL table with a varchar column as foreign key - mysql

Question

I am trying to create a table with a varchar column as foreign key but MySql gives me an error while creating the table. My query is like this:
CREATE TABLE network_classes (
id TINYINT(1) UNSIGNED NOT NULL AUTO_INCREMENT,
category VARCHAR(80) NOT NULL,
PRIMARY KEY(id),
KEY `key_1` (`id`,`category`)
)
ENGINE=InnoDB;
CREATE TABLE networks (
id TINYINT(3) UNSIGNED NOT NULL AUTO_INCREMENT,
name VARCHAR(100) NOT NULL,
category VARCHAR(80) NOT NULL,
director_id TINYINT(3) UNSIGNED NULL,
director_name VARCHAR(100) NULL,
description VARCHAR(1000) NULL,
last_modified TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP,
user_id SMALLINT UNSIGNED NULL,
PRIMARY KEY(id),
KEY `networks_fk1` (`category`),
CONSTRAINT `networks_fk1` FOREIGN KEY (`category`) REFERENCES `network_classes` (`category`) ON DELETE NO ACTION,
INDEX networks_index2471(name),
INDEX networks_index2472(director_id, director_name)
)
ENGINE=InnoDB;
and I get this error:
[Err] 1215 - Cannot add foreign key constraint
I am using MySQL 5.6.12. How can I rewrite my query to fix it?

Answer 1

You can only have a foreign key referencing a unique field. Modify your network_classes table so that the category field is unique, like below
CREATE TABLE network_classes (
id TINYINT(1) UNSIGNED NOT NULL AUTO_INCREMENT,
category VARCHAR(80) NOT NULL,
PRIMARY KEY(id),
UNIQUE KEY `category_UNIQUE` (`category`),
KEY `key_1` (`id`,`category`)
)
ENGINE=InnoDB;
CREATE TABLE networks (
id TINYINT(3) UNSIGNED NOT NULL AUTO_INCREMENT,
name VARCHAR(100) NOT NULL,
category VARCHAR(80) NOT NULL,
director_id TINYINT(3) UNSIGNED NULL,
director_name VARCHAR(100) NULL,
description VARCHAR(1000) NULL,
last_modified TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP,
user_id SMALLINT UNSIGNED NULL,
PRIMARY KEY(id),
KEY `networks_fk1` (`category`),
CONSTRAINT `networks_fk1` FOREIGN KEY (`category`) REFERENCES `network_classes` (`category`) ON DELETE NO ACTION,
INDEX networks_index2471(name),
INDEX networks_index2472(director_id, director_name)
)
ENGINE=InnoDB;
You should then be able to add the foreign key you want

Answer 2

column types in the table and the referenced table do not match for
constraint
Why 2 varchar columns with same size not match in type? And of course the answer is obvious collation. Turns out that in the new table the column was UTF-8 instead of ASCII as in the referenced table. Changed to ascii and done.

Answer 3

The target of a FOREIGN KEY constraint needs to be indexed. Usually, it is a PRIMARY KEY so this isn't an issue, but in your case it's not (although it's part of a composite key, that's not enough)
Create an index on your network_classes.category field:
CREATE INDEX category_idx ON network_classes(category);
Then re-create your networks table.

Answer 4

For me, it was the charset that was missing. Just providing an example for reference.
CREATE TABLE `client` (
id int NOT NULL,
name varchar(255) NOT NULL,
email varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL,
created_at datetime NOT NULL,
created_by varchar(50) NOT NULL DEFAULT 'admin',
updated_at datetime NOT NULL,
updated_by varchar(50) NOT NULL DEFAULT 'admin',
PRIMARY KEY (id),
UNIQUE KEY name_UNIQUE (name),
KEY name_idx (name)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
create table `usecase` (`id` int NOT NULL AUTO_INCREMENT, `client` varchar(255) NOT NULL, `name` varchar(255), `description` varchar(1000), `rule_file` varchar(255), `parsed_rule_file` varchar(255), `archived` BOOLEAN DEFAULT 0, `state` ENUM('INITIATED','PARSED','UPLOADED', 'COMPLETE','FAILED'), `digest` varchar(255), `created_by` varchar(128) DEFAULT NULL, `created_at` datetime(3) NOT NULL DEFAULT CURRENT_TIMESTAMP(3), `updated_by` varchar(128) DEFAULT NULL, `updated_at` datetime(3) DEFAULT NULL, PRIMARY KEY (`id`), UNIQUE KEY `uniq_usecase_id` (`client`,`name`), FOREIGN KEY (`client`) REFERENCES client(`name`))ENGINE=InnoDB DEFAULT CHARSET=latin1;
I had missed the line
DEFAULT CHARSET=latin1

Answer 5

in
CONSTRAINT `networks_fk1` FOREIGN KEY (`category`)
REFERENCES `network_classess` (`category`) ON DELETE NO ACTION,
you have used network_classess instead of network_classes (as in your create table script), so that table not exists.
EDIT
Name of your constraint is the same of key (networks_fk1) change ones.
I read better your DDL.
Your table network_classes has a primary key ID, so is correct put as foreign key a field to link your id field, the category field mustn't appear in your table network, but I think you must put fk_network_class (as int) linked to id (of network_classes)