Can table columns with a Foreign Key be NULL?

Question

I have a table which has several ID columns to other tables.

I want a foreign key to force integrity only if I put data in there. If I do an update at a later time to populate that column, then it should also check the constraint.

(This is likely database server dependant, I'm using MySQL & InnoDB table type)

I believe this is a reasonable expectation, but correct me if I am wrong.

Answer 1

Yes, you can enforce the constraint only when the value is not NULL. This can be easily tested with the following example:

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 INTO child (id, parent_id) VALUES (1, NULL); -- Query OK, 1 row affected (0.01 sec)   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`)) 

The first insert will pass because we insert a NULL in the parent_id. The second insert fails because of the foreign key constraint, since we tried to insert a value that does not exist in the parent table.

Answer 2

I found that when inserting, the null column values had to be specifically declared as NULL, otherwise I would get a constraint violation error (as opposed to an empty string).