Question

Say I have the following tables which implement a many to many relationship between parts and objects, where the link table has a unique key pair (object_id,part_id) such that there are no duplicate rows:

Part table:

part_id | part_name | part_zone
--------+-----------+----------  
0       | part1.0   | top  
1       | part2     | bottom  
2       | part1.1   | top
3       | part3     | top

Object table:

object_id | object_name
----  ----+-----------
0         | object1
1         | object2
2         | object3
3         | object4

Link table:

object_id | part_id
------  --+---------
0         | 0
0         | 1
1         | 1
1         | 2

There is a further constraint in that there should only be one of each part_zone in an object. So if the part_zone column were to be moved to the link table and the key changed to the unique pair (object_id,part_zone) then this would be satisfied. I don’t want to do this however since part_zone should be in the part table. In my head I’m looking to keep the tables as are they are, but use the (object_id,part_zone) unique key where part_zone is pulled from the part table via a join on part_id.

Hopefully my intention is clear, but I’m not too sure this is necessarily the best implementation or I’m using the right terminology for what I’m after.

Thanks in advance to anyone who can offer any insight.

Answer 1

You could add part_zone into Link, with a cascading compound foreign key constraint referencing (part_zone, part_id) in the Part table; that way, you can define a UNIQUE constraint on (object_id, part_zone) as suggested whilst still being able to maintain part_zone from the Part table:

ALTER TABLE Link ADD part_zone varchar(10);  -- should be same size as in Part
ALTER TABLE Part ADD INDEX (part_zone, part_id);

UPDATE Link JOIN Part USING (part_id) SET Link.part_zone = Part.part_zone;

ALTER TABLE Link
  ADD FOREIGN KEY (part_zone, part_id) REFERENCES Part (part_zone, part_id)
    ON UPDATE CASCADE,
  ADD UNIQUE INDEX (object_id, part_zone)
;

You could even define triggers on INSERT and UPDATE to populate the values in Link so that you need not do it manually:

CREATE TRIGGER link_insert BEFORE INSERT ON Link FOR EACH ROW
  SET NEW.part_zone = (SELECT part_zone FROM Part WHERE part_id = NEW.part_id);

CREATE TRIGGER link_update BEFORE UPDATE ON Link FOR EACH ROW
  SET NEW.part_zone = (SELECT part_zone FROM Part WHERE part_id = NEW.part_id);