MYSQL/PHP find the most common item associate with a given item

Question

I have thousands of user generated wish lists of items

the table is something like

collectionId |  itemdId  | user_id
-----------------------------------         


        

Answer 1

Start with a query that gets all the collections that contain the item you selected:

SELECT collectionId
FROM wishLists
WHERE itemId = 876

From this, you want to get all the other itemIds in those collections.

SELECT itemId
FROM wishLists
WHERE collectionId IN (above query)
AND itemId != 876

This can be rewritten as a join:

SELECT a.itemId
FROM wishLists AS a
JOIN wishLists AS b ON a.collectionId = b.collectionId
WHERE a.itemId != 876 AND b.itemId = 876

Now you can count the repetitions of this to find the most common ones:

SELECT a.itemId
FROM wishLists AS a
JOIN wishLists AS b ON a.collectionId = b.collectionId
WHERE a.itemId != 876 AND b.itemId = 876
GROUP BY a.itemId
ORDER BY COUNT(*) DESC

Add a LIMIT n clause at the end to show the top n items.