Can Foreign Key Columns Allow NULL Values While Maintaining Data Integrity?

Can foreign key columns be empty?

Can a table column containing a foreign key be set to NULL? This is particularly important in situations where data integrity must be maintained but the foreign key column may not always have a value.

Answer: Conditional constraint enforcement

Yes, it is possible to enforce foreign key constraints only if the value is not NULL. This ensures data integrity while allowing null values ​​in foreign key columns.

To illustrate this, consider the following MySQL sample code:

CREATE DATABASE t;
USE t;

CREATE TABLE parent (
  id INT NOT NULL,
  PRIMARY KEY (id)
) ENGINE=INNODB;

CREATE TABLE child (
  id INT NULL, 
  parent_id INT NULL,
  FOREIGN KEY (parent_id) REFERENCES parent(id)
) ENGINE=INNODB;

Insert operation with NULL parent_id will succeed:

INSERT INTO child (id, parent_id) VALUES (1, NULL);

However, inserting a value in parent_id that does not exist in the parent table will cause a foreign key constraint violation:

INSERT INTO child (id, parent_id) VALUES (2, 1);

-- ERROR 1452 (23000): Cannot add or update a child row: a foreign key 
-- constraint fails (`t/child`, CONSTRAINT `child_ibfk_1` FOREIGN KEY
-- (`parent_id`) REFERENCES `parent` (`id`))

This indicates that foreign key constraints are only enforced if the parent_id column contains a valid reference, effectively allowing null values ​​to avoid the constraint.

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