How to handle association table and cascade constraints in MYSQL - mysql

Question

I think this is a common problem but I was not able to find correct keywords to perform a search on StackOverflow.
In mysql, I have two entites : organization and scale.
One organization can only have one scale but a scale can belong to many organizations.
So I created a third entity organization_scale.
for instance :
organization
------------
org_id | name
1 | Mickey
2 | Donald
3 | Dingo
scale
----------
sc_id | name
1 | miniScale
2 | maxiScale
organization_scale
--------------
org_id | sc_id
1 | 1
2 | 1
3 | 2
Both fields of the organization_scale entity are foreign keys + cascade on update and delete
The problem is the following:
If I delete from organization where org_id = 3,the third line from organization_scale are properly deleted (since org_id = 3 does not exist anymore)
BUT
the scale with sc_id = 2 is not deleted. And I do not understand why. No one references to this scale, and this should be removed otherwise, with a big amount of data, a lot of "scales" will be orphean.
(same thing happens if I do not use an extra table, and I add a scale_id column directly in organization)

Answer 1

I think the problem is this: The ON DELTE CASCADE in the definition of the organization_scale table only works in one direction: when deleting something in scale or organization then don't ask and delete the rows in organization_scale.
But you seem to want the other direction: When deleting something in organization_scale delete the row in scale if there is no reference to it anymore.
This works for me: I added a trigger on organization.
DROP DATABASE IF EXISTS test;
CREATE DATABASE test;
USE test;
CREATE TABLE organization (org_id INT UNSIGNED PRIMARY KEY, name VARCHAR(255));
CREATE TABLE scale (sc_id INT UNSIGNED PRIMARY KEY, name VARCHAR(255));
CREATE TABLE organization_scale (org_id INT UNSIGNED, sc_id INT UNSIGNED,
FOREIGN KEY (org_id)
REFERENCES organization(org_id)
ON UPDATE CASCADE ON DELETE CASCADE,
FOREIGN KEY (sc_id)
REFERENCES scale(sc_id)
ON UPDATE CASCADE ON DELETE CASCADE
);
CREATE TRIGGER delete_unused_sc_id AFTER DELETE ON organization FOR EACH ROW DELETE FROM scale WHERE NOT EXISTS (SELECT * FROM organization_scale WHERE organization_scale.sc_id = scale.sc_id);
INSERT INTO organization VALUES
(1,"Mickey"),
(2,"Donald"),
(3,"Dingo");
INSERT INTO scale VALUES
(1,"miniScale"),
(2,"maxiScale");
INSERT INTO organization_scale VALUES
(1,1),
(2,1),
(3,2);
SELECT * FROM organization_scale;
SELECT * FROM scale;
DELETE FROM organization WHERE org_id=3;
SELECT * FROM organization_scale;
SELECT * FROM scale;
DROP DATABASE test;