1949. Strong Friendship

https://leetcode.com/problems/strong-friendship

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.
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 an SQL query 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 query result format is in the following example:

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        |
+----------+----------+
Result table:
+----------+----------+---------------+
| user1\_id | user2\_id | common\_friend |
+----------+----------+---------------+
| 1        | 2        | 4             |
| 1        | 3        | 3             |
+----------+----------+---------------+
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).

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