Enforce order on composite primary key - mysql

Question

Say I have the following table
item_a_id, item_b_id, value
Where item_a_id and item_b_id are a composite primary key. In my example a,b and b,a are equivalent. Therefore I want to ensure that item_a_id < item_b_id. Obviously the application logic will enforce this but is there a way to ensure the database does too?

Answer 1

In a reasonably current version of MySql you can use triggers to emulate a check constraint that produces the desired behavior.

Answer 2

Well, in your case, you could use trigger to check values before insert/update and swap it to ensure item_a_id will always less than item_b_id.
Assuming the table name is item_links, you could try this:
DELIMITER |
CREATE TRIGGER ensure_a_b_before_insert BEFORE INSERT ON item_links
FOR EACH ROW
BEGIN
IF NEW.item_a_id > NEW.item_b_id THEN
SET #tmp = NEW.item_b_id;
SET NEW.item_b_id = NEW.item_a_id;
SET NEW.item_a_id = #tmp;
END IF;
END;
|
CREATE TRIGGER ensure_a_b_before_update BEFORE UPDATE ON item_links
FOR EACH ROW
BEGIN
IF NEW.item_a_id > NEW.item_b_id THEN
SET #tmp = NEW.item_b_id;
SET NEW.item_b_id = NEW.item_a_id;
SET NEW.item_a_id = #tmp;
END IF;
END;
|
DELIMITER ;
Here's what I got when I test inserting:
mysql> INSERT INTO `item_links` (`item_a_id`, `item_b_id`, `value`)
-> VALUES ('1', '2', 'a')
-> , ('3', '2', 'b')
-> , ('4', '1', 'c');
Query OK, 3 rows affected (0.01 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> SELECT * FROM `item_links`;
+-----------+-----------+-------+
| item_a_id | item_b_id | value |
+-----------+-----------+-------+
| 1 | 2 | a |
| 2 | 3 | b |
| 1 | 4 | c |
+-----------+-----------+-------+
3 rows in set (0.00 sec)
Update works, too:
mysql> UPDATE `item_links`
-> SET `item_a_id` = 100, `item_b_id` = 20
-> WHERE `item_a_id` = 1 AND `item_b_id` = 2;
Query OK, 1 row affected (0.03 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> SELECT * FROM `item_links`;
+-----------+-----------+-------+
| item_a_id | item_b_id | value |
+-----------+-----------+-------+
| 20 | 100 | a |
| 2 | 3 | b |
| 1 | 4 | c |
+-----------+-----------+-------+
3 rows in set (0.00 sec)