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Formatted question description: https://leetcode.ca/all/1892.html
1892. Page Recommendations II
Level
Hard
Description
Table: Friendship
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| user1_id | int |
| user2_id | int |
+---------------+---------+
(user1_id, user2_id) is the primary key for this table.
Each row of this table indicates that the users user1_id and user2_id are friends.
Table: Likes
+-------------+---------+
| Column Name | Type |
+-------------+---------+
| user_id | int |
| page_id | int |
+-------------+---------+
(user_id, page_id) is the primary key for this table.
Each row of this table indicates that user_id likes page_id.
You are implementing a page recommendation system for a social media website. Your system will recommend a page to user_id if the page is liked by at least one friend of user_id and is not liked by user_id.
Write an SQL query to find all the possible page recommendations for every user. Each recommendation should appear as a row in the result table with these columns:
user_id: The ID of the user that your system is making the recommendation to.page_id: The ID of the page that will be recommended to user_id.friends_likes: The number of the friends of user_id that like page_id.
Return result table in any order.
The query result format is in the following example:
Friendship table:
+----------+----------+
| user1_id | user2_id |
+----------+----------+
| 1 | 2 |
| 1 | 3 |
| 1 | 4 |
| 2 | 3 |
| 2 | 4 |
| 2 | 5 |
| 6 | 1 |
+----------+----------+
Likes table:
+---------+---------+
| user_id | page_id |
+---------+---------+
| 1 | 88 |
| 2 | 23 |
| 3 | 24 |
| 4 | 56 |
| 5 | 11 |
| 6 | 33 |
| 2 | 77 |
| 3 | 77 |
| 6 | 88 |
+---------+---------+
Result table:
+---------+---------+---------------+
| user_id | page_id | friends_likes |
+---------+---------+---------------+
| 1 | 77 | 2 |
| 1 | 23 | 1 |
| 1 | 24 | 1 |
| 1 | 56 | 1 |
| 1 | 33 | 1 |
| 2 | 24 | 1 |
| 2 | 56 | 1 |
| 2 | 11 | 1 |
| 2 | 88 | 1 |
| 3 | 88 | 1 |
| 3 | 23 | 1 |
| 4 | 88 | 1 |
| 4 | 77 | 1 |
| 4 | 23 | 1 |
| 5 | 77 | 1 |
| 5 | 23 | 1 |
+---------+---------+---------------+
Take user 1 as an example:
- User 1 is friends with users 2, 3, 4, and 6.
- Recommended pages are 23 (user 2 liked it), 24 (user 3 liked it), 56 (user 3 liked it), 33 (user 6 liked it), and 77 (user 2 and user 3 liked it).
- Note that page 88 is not recommended because user 1 already liked it.
Another example is user 6:
- User 6 is friends with user 1.
- User 1 only liked page 88, but user 6 already liked it. Hence, user 6 has no recommendations.
You can recommend pages for users 2, 3, 4, and 5 using a similar process.
Solution
Select two users user1_id and user2_id from Friendship, and select the pages liked by user2_id but not liked by user1_id. Use inner join to obtain the pages recommended to user1_id.
# Write your MySQL query statement below
select f.user1_id as user_id, l.page_id, count(*) as friends_likes
from (
select user1_id, user2_id from Friendship union select user2_id, user1_id from Friendship
) as f
inner join Likes l
on f.user2_id = l.user_id
where not exists (
select * from Likes where user_id = f.user1_id and page_id = l.page_id
)
group by f.user1_id, l.page_id;