Are multiple foreign keys in a single field possible?

Question

I want to know if there is a way to have multiple values in a single field in a MySQL database where each value is a foreign key referencing one other table.

I am de

Answer 1

A single field cannot be a foreign key to more than one field in another table. It's just not possible. Since your foreign table has a composite key, your table in question would have to have the same fields as well.

Answer 2

What you typically do is set up a many to many relationship with an intermediate linking table. Some thing like the following:

CREATE TABLE product (
  `id` integer AUTO_INCREMENT NOT NULL,
  -- other cols --
  PRIMARY KEY (`id`)
);

CREATE TABLE certification (
  `id` integer AUTO_INCREMENT NOT NULL,
  -- other cols --
  PRIMARY KEY (`id`)
);

CREATE TABLE product_certification (
   `product_id` integer NOT NULL,
   `certification_id` integer NOT NULL,
   PRIMARY KEY (`product_id`, `certification_id`),
   CONSTRAINT `product_id_product_id` 
     FOREIGN KEY (`product_id`) 
     REFERENCES `product` (`id`) ON DELETE CASCADE,
   CONSTRAINT `certification_id_certification_id` 
     FOREIGN KEY (`certification_id`) 
     REFERENCES `certification` (`id`) ON DELETE CASCADE
);

Answer 3

If I understand you correctly, the relation product : product_certification is 1:M you can create a foreign key from product_certification to products via product_id, instead of having product_certification_id in the products table (which is invalid, since the product can have more than 1 certification)