1917. Leetcodify Friends Recommendations

https://leetcode.com/problems/leetcodify-friends-recommendations

Description

Table: Listens

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| user\_id     | int     |
| song\_id     | int     |
| day         | date    |
+-------------+---------+
There is no primary key for this table. It may contain duplicates.
Each row of this table indicates that the user user\_id listened to the song song\_id on the day day.

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.
Note that user1\_id < user2\_id.

Write an SQL query to recommend friends to Leetcodify users. We recommend user x to user y if:

• Users x and y are not friends, and

• Users x and y listened to the same three or more different songs on the same day.

Note that friend recommendations are unidirectional, meaning if user x and user y should be recommended to each other, the result table should have both user x recommended to user y and user y recommended to user x. Also, note that the result table should not contain duplicates (i.e., user y should not be recommended to user x multiple times.).

Return the result table in any order.

The query result format is in the following example:

Listens table:
+---------+---------+------------+
| user\_id | song\_id | day        |
+---------+---------+------------+
| 1       | 10      | 2021-03-15 |
| 1       | 11      | 2021-03-15 |
| 1       | 12      | 2021-03-15 |
| 2       | 10      | 2021-03-15 |
| 2       | 11      | 2021-03-15 |
| 2       | 12      | 2021-03-15 |
| 3       | 10      | 2021-03-15 |
| 3       | 11      | 2021-03-15 |
| 3       | 12      | 2021-03-15 |
| 4       | 10      | 2021-03-15 |
| 4       | 11      | 2021-03-15 |
| 4       | 13      | 2021-03-15 |
| 5       | 10      | 2021-03-16 |
| 5       | 11      | 2021-03-16 |
| 5       | 12      | 2021-03-16 |
+---------+---------+------------+
Friendship table:
+----------+----------+
| user1\_id | user2\_id |
+----------+----------+
| 1        | 2        |
+----------+----------+
Result table:
+---------+----------------+
| user\_id | recommended\_id |
+---------+----------------+
| 1       | 3              |
| 2       | 3              |
| 3       | 1              |
| 3       | 2              |
+---------+----------------+
Users 1 and 2 listened to songs 10, 11, and 12 on the same day, but they are already friends.
Users 1 and 3 listened to songs 10, 11, and 12 on the same day. Since they are not friends, we recommend them to each other.
Users 1 and 4 did not listen to the same three songs.
Users 1 and 5 listened to songs 10, 11, and 12, but on different days.
Similarly, we can see that users 2 and 3 listened to songs 10, 11, and 12 on the same day and are not friends, so we recommend them to each other.

ac