Question

I’m creating a ‘similar items’ link table.

i have a 2 column table. both columns contains product ids.

CREATE TABLE IF NOT EXISTS `prod_similar` (
  `id_a` int(11) NOT NULL,
  `id_b` int(11) NOT NULL
)    

INSERT INTO `prod_similar` (`id_a`, `id_b`) VALUES
(5, 10),
(5, 15),
(10, 13),
(10, 14),
(14, 5),
(14, 13);

I want to select 3 similar products, favouring products where the id is in the first col, ‘id_a’

SELECT * FROM prod_similar WHERE id_a={$id} OR id_b={$id}
ORDER BY column(?) 
LIMIT 3

Answer 1

I assume you have other columns as well

(SELECT 1 favouring, id_a id, [other columns]
FROM prod_similar
WHERE id_a = {$id})
UNION
(SELECT 2 favouring, id_b id, [other columns]
FROM prod_similar
WHERE id_b = {$id})
ORDER BY favouring, id
LIMIT 3;

In case you don’t mind duplicates or there are none between id_a and id_b you can do UNION ALL instead which is considerably faster.

Unions are indication of denormalized data, denormalized data improves speed of certain queries and reduces speed of others (such as this).