Re: exclusion constraint question

Hey Rhys,

I think you might be better served doing something like

CREATE TABLE shelve_items(
id bigint PRIMARY KEY,
shelf_id bigint,
column_name VARCHAR,
mug_id bigint UNIQUE
)

and then putting a unique index on (shelf_id, column_name):

CREATE UNIQUE INDEX shelve_items_id_c_name on shelve_items (shelf_id, column_name)

The first row of your shelf would then look like:

INSERT INTO shelve_items VALUES (1, 1, left, 7)
INSERT INTO shelve_items VALUES (2, 1, middle, 2)
INSERT INTO shelve_items VALUES (3, 1, right, 1)

This would also allow you to scale to shelves that are n items wide (maybe at that point you could use an id for the columns instead of names)

Does this work for you?

Best,
Emanuel

> On 8 Mar 2025, at 20:01, Rhys A.D. Stewart <rhys(dot)stewart(at)gmail(dot)com> wrote:
>
> Greetings All,
>
> I have the following table:
>
> CREATE TABLE shelves(
> shelf_id bigint PRIMARY KEY,
> l_mug_id bigint UNIQUE,
> c_mug_id bigint UNIQUE,
> r_mug_id bigint UNIQUE,
> CHECK (l_mug_id <> c_mug_id AND l_mug_id <> r_mug_id AND c_mug_id
> <> r_mug_id),
> EXCLUDE USING gist (l_mug_id WITH <>, c_mug_id WITH <>, r_mug_id
> WITH <>) -- Not working as expected (or my expectations are wrong).
> );
>
> And some data:
>
> INSERT INTO shelves VALUES (1, 7, 2, 1);
> INSERT INTO shelves VALUES (2, 3, null, null);
> INSERT INTO shelves VALUES (3, null, 1, 4);
> INSERT INTO shelves VALUES (4, 4, 5, null);
>
> Mugs on shelves, fascinating. A mug_id can only appear once in the
> entire table. The check constraint handles not having the same mug_id
> in each row and the unique constraints does the same for the column.
> But how do I get around checking multiple columns for the same mug_id.
> I'm thinking an exclusion constraint, but (a) I do not know if I am
> overthinking it and (b) the exclusion constraint I have does not work
> as expected, or my expectations are way off.
>
> Any suggestions would be appreciated.
>
> Regards,
>
> Rhys
> Peace & Love | Live Long & Prosper
>
>