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

Some help:

Assuming you are looking the most similar to goods#1

SELECT a.*  
FROM (SELECT * FROM goods WHERE id <> 1) a 
LEFT JOIN (SELECT z.goods_id, count(*) as total
          FROM links z
          WHERE z.goods_id <> 1 AND
          z.tag_id in (SELECT DISTINCT tag_id from links where goods_id = 1)
          GROUP BY z.goods_id) b 
ON a.id = b.goods_id
ORDER by b.total DESC

However, I think you can try something a bit different. Instead of ordering by the number of common tags, you can sort by the ratio of common tags. With this you will avoid the fact that the products with more tags will appear always at the top of the rankings, even if the relative common tags are not many.

Answer 2

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;

Answer 3

When you want to show the goods with goods id = 2

SELECT DISTINCT
  goods.*
FROM
  goods
  LEFT JOIN links ON links.goods_id = goods.id
WHERE links.tag_id IN (SELECT links.tag_id 
                       FROM links
                       WHERE links.goods_id = 2)

when you did not include goods_id = 2

SELECT DISTINCT
  goods.*
FROM
  goods
  LEFT JOIN links ON links.goods_id = goods.id
WHERE links.goods_id != 2 AND links.tag_id IN (SELECT links.tag_id 
                       FROM links
                       WHERE links.goods_id = 2)

can see on http://sqlfiddle.com/#!2/0fb60/38