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1949. Strong Friendship

Description

Table: Friendship

+-------------+------+
| Column Name | Type |
+-------------+------+
| user1_id    | int  |
| user2_id    | int  |
+-------------+------+
(user1_id, user2_id) is the primary key (combination of columns with unique values) 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.

 

A friendship between a pair of friends x and y is strong if x and y have at least three common friends.

Write a solution to find all the strong friendships.

Note that the result table should not contain duplicates with user1_id < user2_id.

Return the result table in any order.

The result format is in the following example.

 

Example 1:

Input: 
Friendship table:
+----------+----------+
| user1_id | user2_id |
+----------+----------+
| 1        | 2        |
| 1        | 3        |
| 2        | 3        |
| 1        | 4        |
| 2        | 4        |
| 1        | 5        |
| 2        | 5        |
| 1        | 7        |
| 3        | 7        |
| 1        | 6        |
| 3        | 6        |
| 2        | 6        |
+----------+----------+
Output: 
+----------+----------+---------------+
| user1_id | user2_id | common_friend |
+----------+----------+---------------+
| 1        | 2        | 4             |
| 1        | 3        | 3             |
+----------+----------+---------------+
Explanation: 
Users 1 and 2 have 4 common friends (3, 4, 5, and 6).
Users 1 and 3 have 3 common friends (2, 6, and 7).
We did not include the friendship of users 2 and 3 because they only have two common friends (1 and 6).

Solutions

• SQL

• # Write your MySQL query statement below
  WITH
      t AS (
          SELECT
              *
          FROM Friendship
          UNION ALL
          SELECT
              user2_id,
              user1_id
          FROM Friendship
      )
  SELECT
      t1.user1_id,
      t1.user2_id,
      COUNT(1) AS common_friend
  FROM
      t AS t1
      JOIN t AS t2 ON t1.user2_id = t2.user1_id
      JOIN t AS t3 ON t1.user1_id = t3.user1_id
  WHERE t3.user2_id = t2.user2_id AND t1.user1_id < t1.user2_id
  GROUP BY t1.user1_id, t1.user2_id
  HAVING COUNT(1) >= 3;
  
  

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