what is the problem with my tables in mysql? - mysql

Question

I'm going to create 3 table. table "post" and "taxonomy" are connected to "taxonomy_relationship" table with primary key and foreign key but i don't know why do i get this error:
1005 - Can't create table taxonomy_relationship (errno: 150 "Foreign key constraint is incorrectly formed")
CREATE TABLE `post`(
id int not null AUTO_INCREMENT,
title varchar(255) not null,
content TEXT not null,
PRIMARY KEY(id)
);
CREATE TABLE `taxonomy`(
id int not null AUTO_INCREMENT,
tax_title varchar(255) not null,
type varchar(255) not null,
PRIMARY KEY(id, tax_title)
);
CREATE TABLE taxonomy_relationship(
id INT AUTO_INCREMENT,
post_id int not null,
title varchar(255) not null,
PRIMARY KEY(id),
FOREIGN KEY (post_id)
REFERENCES post(id)
ON DELETE CASCADE
ON UPDATE CASCADE
FOREIGN KEY (title)
REFERENCES taxonomy(tax_title)
ON DELETE CASCADE
ON UPDATE CASCADE
);
when i use "SHOW ENGINE INNODB STATUS" to show error details it returns:
>
2020-01-19 16:08:31 0x2fb8 Error in foreign key constraint of table blog.taxonomy_relationship:
FOREIGN KEY (title)
REFERENCES taxonomy(tax_title)
ON DELETE CASCADE
ON UPDATE CASCADE
):
Cannot find an index in the referenced table where the
referenced columns appear as the first columns, or column types
in the table and the referenced table do not match for constraint.
Note that the internal storage type of ENUM and SET changed in
tables created with >= InnoDB-4.1.12, and such columns in old tables
cannot be referenced by such columns in new tables.
Please refer to https://mariadb.com/kb/en/library/foreign-keys/ for correct foreign key definition.
Create table blog.taxonomy_relationship with foreign key constraint failed. There is no index in the referenced table where the referenced columns appear as the first columns near 'FOREIGN KEY (title)
REFERENCES taxonomy(tax_title)
ON DELETE CASCADE
ON UPDATE CASCADE
)'.

Answer 1

You should to add index KEY title_idx (tax_title) on taxonomy table :
CREATE TABLE `taxonomy` (
`id` int NOT NULL AUTO_INCREMENT,
`tax_title` varchar(255) NOT NULL,
`type` varchar(255) NOT NULL,
PRIMARY KEY (`id`),
KEY `title_idx` (`tax_title`)
);
After you can create foreign key:
CREATE TABLE taxonomy_relationship(
id INT AUTO_INCREMENT,
post_id int not null,
title varchar(255) not null,
PRIMARY KEY(id),
FOREIGN KEY (post_id)
REFERENCES post(id)
ON DELETE CASCADE
ON UPDATE CASCADE,
FOREIGN KEY (title)
REFERENCES taxonomy(tax_title)
ON DELETE CASCADE
ON UPDATE CASCADE
);
Another way you can duplicate title in taxonomy_relationship but link it by id field:
CREATE TABLE taxonomy_relationship(
id INT AUTO_INCREMENT,
post_id int not null,
taxonomy_id int not null,
PRIMARY KEY(id),
FOREIGN KEY (post_id)
REFERENCES post(id)
ON DELETE CASCADE
ON UPDATE CASCADE,
FOREIGN KEY (taxonomy_id)
REFERENCES taxonomy(id)
ON DELETE CASCADE
ON UPDATE CASCADE
);