# Exercise 24：1917.Leetcodify Friends Recommendations

## 1.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.

Example 1:

```
Input: 
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        |
+----------+----------+
Output: 
+---------+----------------+
| user_id | recommended_id |
+---------+----------------+
| 1       | 3              |
| 2       | 3              |
| 3       | 1              |
| 3       | 2              |
+---------+----------------+
Explanation: 
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.
```

## 2.Create Table and insert into values

```sql
create table if not exists leetcode.ex_1917_listens
(user_id string ,  song_id string, day date ) stored as orc ;

INSERT INTO table leetcode.ex_1917_listens VALUES
('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') ; 

create table if not exists leetcode.ex_1917_friendship
(user1_id string ,  user2_id string ) stored as orc ;

INSERT INTO table leetcode.ex_1917_friendship VALUES
('1','2') ; 
```

{% hint style="info" %}
In order to prevent the stability of big data clusters. Similar non-congruent joins (non-inner joins) are forbidden, and SemanticException Cartesian products are disabled.

**IF we want non-congruent joins, Add two settings before the query：**

`set hive.strict.checks.cartesian.product=flase;`

`set hive.mapred.mode=nonstrict;`

Check this [link](https://blog.spacepatroldelta.com/a?ID=01600-6c29f541-c4cb-4747-b74f-896ec2862aa6) for more detail.
{% endhint %}