How to avoid adding duplicate foreign key constraints - mysql

Question

I would to know if it is possible to avoid adding several times the same foreign key constraint?
Example: if I execute 3 times the query below, the constraint will exist with 3 times in phpmyadmin... It would be great that it would be rejected the second and third time I apply that query.
ALTER TABLE `my_items_details`
ADD FOREIGN KEY (`items_id`) REFERENCES `my_items`(`item_id`)
ON DELETE RESTRICT
ON UPDATE CASCADE;

Answer 1

You can give a explicit name to the foreign key, instead of letting MySQL assigning a default name for you.
ALTER TABLE `my_items_details`
ADD FOREIGN KEY `my_foreign_key` (`item_id`) REFERENCES `my_items`(`item_id`)
ON DELETE RESTRICT
ON UPDATE CASCADE;
Since two objects of the same type cannot have the same name, this query will generate an error the second time you run it:
Error: ER_FK_DUP_NAME: Duplicate foreign key constraint name 'my_foreign_key'
Demo on DB Fiddle

Answer 2

To prevent duplicate foreign key values in a table in a database, you can define a unique index or unique constraint on the foreign key field in the table.
In PHPMyAdmin, you can create a unique index or constraint on a foreign key field by following these steps:
Select the table from the list of tables in the left-hand panel.
Click on the "Structure" tab.
Scroll down to the field that you want to define the unique index or constraint on.
Click on the "Index" drop-down menu for the field, and select "Unique".
Alternatively, you can use the following SQL query to create a unique index on a foreign key field called item_id in a table called items:
ALTER TABLE items ADD UNIQUE (item_id);