Can a Single Database Column Reference Multiple Foreign Keys?

Multi-Referencing Foreign Keys within a Database Column

In database design, foreign key constraints play a crucial role in maintaining referential integrity and ensuring data accuracy. Typically, a foreign key column references a primary key column in another table, establishing a one-to-many or many-to-many relationship between records. However, the question arises: Can we define a single column that acts as a foreign key to multiple tables?

Can a Column Reference Multiple Foreign Keys?

The answer is no. According to database principles, it is not permissible to have a single column reference multiple foreign keys simultaneously. This restriction is enforced by all major relational database management systems (RDBMSs).

Explanation

The primary purpose of a foreign key is to establish a direct link between records in different tables. Each foreign key value should uniquely identify a row in the referenced table. Attempting to reference multiple foreign keys in a single column would violate this foundational principle.

Alternatives to Multi-Referencing Foreign Keys

If you need to establish relationships between records in multiple tables, there are alternative approaches to consider:

1. Join Table: Create a separate table that contains the primary keys of the related tables. This join table serves as an intermediary to connect records across multiple tables.
2. Composite Key: Define a primary key that consists of multiple columns. This allows you to create foreign key constraints that reference multiple columns simultaneously.

Example Using Composite Key

Consider the example provided in the question:

CREATE TABLE pdf_created (
    `id` INT(10) UNSIGNED NOT NULL UNIQUE AUTO_INCREMENT,
    `pdf_id` INT(10) NOT NULL,
    `item_type` INT(3) UNSIGNED NOT NULL,
    `item_id` INT(10) UNSIGNED NOT NULL,
    PRIMARY KEY (`id`, `item_type`, `item_id`)
);

CREATE TABLE `header` (
    `id` INT(10) UNSIGNED NOT NULL UNIQUE AUTO_INCREMENT,
    `title` VARCHAR(255)
);

CREATE TABLE `service` (
    `id` INT(10) UNSIGNED NOT NULL UNIQUE AUTO_INCREMENT,
    `desc` VARCHAR(65535) NOT NULL
);

CREATE TABLE `product` (
    `id` INT(10) UNSIGNED NOT NULL UNIQUE AUTO_INCREMENT,
    `desc` VARCHAR(65535) NOT NULL
);

In this schema, the item_id column in the pdf_created table can reference both the id column in the product table and the id column in the service table. This is achieved by defining a composite primary key on the pdf_created table using both id and item_type columns.

Conclusion

While it is not directly possible to have a single column reference multiple foreign keys in a relational database, alternative approaches such as join tables or composite keys can be used to establish the necessary relationships between records.

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