sql query to determine the most similar goods by tags

Question

i\'m making an e-store, so i have 3 tables:

1) goods

id      | title
--------+----------- 
1       | Toy car
2       | Toy pony
3       | Do         


        

Answer 1

This query will return all items that have the maximum number of tags in common:

SET @item = 1;

SELECT
  goods_id
FROM
  links
WHERE
  tag_id IN (SELECT tag_id FROM links WHERE goods_id=@item)
  AND goods_id!=@item
GROUP BY
  goods_id
HAVING
  COUNT(*) = (
    SELECT
      COUNT(*)
    FROM
      links
    WHERE
      tag_id IN (SELECT tag_id FROM links WHERE goods_id=@item)
      AND goods_id!=@item
    GROUP BY
      goods_id
    ORDER BY
      COUNT(*) DESC
    LIMIT 1
  )

Please see fiddle here.

Or this one will return all items, even those with no tags in common, ordered by the number of tags in common desc:

SELECT
  goods_id
FROM
  links
WHERE
  goods_id!=@item
GROUP BY
  goods_id
ORDER BY
  COUNT(CASE WHEN tag_id IN (SELECT tag_id FROM links WHERE goods_id=@item) THEN 1 END) DESC;