MySQL Show recommended items - php

Question

I have a news system where users can recommend the item. But now I want that when someone is viewing an item, display a list of other items that have been recommended by people who recommended the item is viewed. Something like "Users who recommended this item also recommended..."
Recommendations table
+----+------+---------+
| id | user | item |
+----+------+---------+
| 1 | 1 | 1 | User 1 recommended item 1
+----+------+---------+
| 2 | 1 | 2 |
+----+------+---------+
| 3 | 2 | 2 |
+----+------+---------+
| 4 | 3 | 3 |
+----+------+---------+
| 5 | 2 | 3 |
+----+------+---------+
Items table
+----+---------+---------+
| id | title | author |
+----+---------+---------+
| 1 |Hello... | me |
+----+---------+---------+
| 2 | Bye... | you |
+----+---------+---------+
| 3 | Hi... | me |
+----+---------+---------+
As you can see the user with ID 1, recommended item 1 and 2 and User 2 also recommended item 2, also recommended item 3.
So when someone is looking at item 2, should be displayed in the list the item 3.
Also when someone is watching item 3, you should see the list the item 2.
Do not know how to do the SQL query, I guess first I have to get the IDs of all users who recommended the article being viewed, and later, check the ID of items most repeated among the selected user IDs.
And then with those IDs of the items, get the item title or author information.
But I have no idea how to make the SQL query in a more optimized way. I would appreciate your supports.

Answer 1

I splitted your problem into 2 tasks:
1.Select all the users, that recommended this item except the current user:
SELECT
recommendations.user
FROM
recommendations
WHERE
recommendations.item=$item_id
AND
recommendations.user!=$user_id
2.Then we use this query as a subquery to get all the items, recommended by those users, except the currently viewed item:
SELECT
items.id,
items.title
FROM
items
INNER JOIN
recommendations
ON
items.id=recommendations.item
WHERE
recommendations.user IN
(
SELECT
recommendations.user
FROM
recommendations
WHERE
recommendations.item=$item_id
AND
recommendations.user!=$user_id
)
AND
items.id!=$item_id
GROUP BY
items.id
3.If you want to show the most recommended items first, you'll need to sort the results by number of users, who recommended the item:
SELECT
items.id,
items.title,
COUNT(*) AS cnt
FROM
items
INNER JOIN
recommendations
ON
items.id=recommendations.item
WHERE
recommendations.user IN
(
SELECT
recommendations.user
FROM
recommendations
WHERE
recommendations.item=$item_id
AND
recommendations.user!=$user_id
)
AND
items.id!=$item_id
GROUP BY
items.id
ORDER BY
cnt DESC

Answer 2

You could certainly accomplish your goal using Joins, but it might be a better idea to have a third table with a one to one relationship for each item and a corresponding recommended item.
This way your Mysql query for recommended items will be much more efficient querying this third table for the source item and returning a result set of all recommended items.
You can then also do a simple query to the recommendations table to get the ids of the people that recommended the items in question.
With small numbers of items, this may seem to be overkill, but as you approach large numbers, the optimization could be significant in the Mysql processing required.