1264. Page Recommendations.sql

-- Write an SQL query to recommend pages to the user with user_id = 1 using the pages that your friends liked. It should not recommend pages you already liked.
                    
SELECT DISTINCT page_id AS recommended_page
FROM Likes l
JOIN Friendship f
ON (f.user1_id = 1 AND l.user_id = f.user2_id) OR (f.user2_id = 1 AND l.user_id = f.user1_id)  -- All friends of user1 = 1 or user2 = 1
WHERE page_id NOT IN (SELECT page_id FROM Likes WHERE user_id = 1) 

/* GROUP BYInput: 
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      |
+---------+---------+
Output: 
+------------------+
| recommended_page |
+------------------+
| 23               |
| 24               |
| 56               |
| 33               |
| 77               |
+------------------+
Explanation: 
User one is friend with users 2, 3, 4 and 6.
Suggested pages are 23 from user 2, 24 from user 3, 56 from user 3 and 33 from user 6.
Page 77 is suggested from both user 2 and user 3.
Page 88 is not suggested because user 1 already likes it.
*/