Question

I have categories and multiple categorization for my Items. How to find, for specific Item, other Items that have same categories, ordered by most categories matching (aka best match)?

My table structure is roughly:

Item Table

ID
Name
...

Category Table

ID
Name
...

Categorization Table

ID
Item_ID
Category_ID
...

To find all Items having similar categories, for example, I use

SELECT `items`.* 
FROM `items` 
INNER JOIN `categorizations` c1
ON c1.`item_id` = `items`.`id`
INNER JOIN `categorizations` c2
ON c2.`item_id` = <Item_ID>
WHERE `c1.`category_id` = c2.`category_id`

Answer 1

This should produce a table of counts of category matches between each pair of items that share at least one category.

select i1.item_id,i2.item_id,count(1)
from items i1
join categorizations c1 on c1.item_id=i1.item_id
join categorizations c2 on c2.category_id=c1.category_id
join items i2 on c2.item_id=i2.item_id
where i1.item_id <> i2.item_id
group by i1.item_id,i2.item_id
order by count(1)

I suspect that it may be a bit slow, though. I don’t have an instance of MySQL at the moment to try it out.