For example, I have a table which has several ID columns to other tables. I want a foreign key to force integrity only if I do put data in there. If I do an update at a later time to populate that column then it will still 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.
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 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