Question

I have an integer column which I would like to add a foreign key constraint to. Only problem is if that column does not have/need a value, by default MySQL puts in a ‘0’ value. This obviously breaks the foreign key constraint as there is no record in the primary table with a PK of 0.

How can I overcome this problem?

Answer 1

You may want to set your foreign key to accept NULL values, and use a NULL instead of a 0 value.

Conceptually, NULL means a missing unknown value. If your row does “not have/need the value”, I believe a NULL fits in perfectly.

And yes, a NULL value would not break your foreign key constraint.

Let’s build a basic example:

CREATE TABLE parents (
   id      int PRIMARY KEY, 
   value   int
) ENGINE = INNODB;

CREATE TABLE children (
   id         int PRIMARY KEY, 
   parent_id  int,
   FOREIGN KEY (parent_id) REFERENCES parent (id)
) ENGINE = INNODB;

Then:

INSERT INTO parents VALUES (1, 100);
Query OK, 1 row affected (0.00 sec)

INSERT INTO children VALUES (1, 1);
Query OK, 1 row affected (0.00 sec)

INSERT INTO children VALUES (2, 0);
ERROR 1452 (23000): A foreign key constraint fails

INSERT INTO children VALUES (2, NULL);
Query OK, 1 row affected (0.00 sec)

SELECT * FROM children;
+----+-----------+
| id | parent_id |
+----+-----------+
|  1 |         1 |
|  2 |      NULL |
+----+-----------+
2 rows in set (0.01 sec)