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1892. Page Recommendations II

English Version

题目描述

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.

 

Table: Likes

+-------------+---------+

| Column Name | Type    |

+-------------+---------+

| user_id     | int     |

| page_id     | int     |

+-------------+---------+

(user_id, page_id) is the primary key for this table.

Each row of this table indicates that user_id likes page_id.

 

You are implementing a page recommendation system for a social media website. Your system will recommended a page to user_id if the page is liked by at least one friend of user_id and is not liked by user_id.

Write an SQL query to find all the possible page recommendations for every user. Each recommendation should appear as a row in the result table with these columns:

  • user_id: The ID of the user that your system is making the recommendation to.
  • page_id: The ID of the page that will be recommended to user_id.
  • friends_likes: The number of the friends of user_id that like page_id.

Return result table in any order.

The query result format is in the following example:

 

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      |

+---------+---------+



Result table:

+---------+---------+---------------+

| user_id | page_id | friends_likes |

+---------+---------+---------------+

| 1       | 77      | 2             |

| 1       | 23      | 1             |

| 1       | 24      | 1             |

| 1       | 56      | 1             |

| 1       | 33      | 1             |

| 2       | 24      | 1             |

| 2       | 56      | 1             |

| 2       | 11      | 1             |

| 2       | 88      | 1             |

| 3       | 88      | 1             |

| 3       | 23      | 1             |

| 4       | 88      | 1             |

| 4       | 77      | 1             |

| 4       | 23      | 1             |

| 5       | 77      | 1             |

| 5       | 23      | 1             |

+---------+---------+---------------+

Take user 1 as an example:

  - User 1 is friends with users 2, 3, 4, and 6.

  - Recommended pages are 23 (user 2 liked it), 24 (user 3 liked it), 56 (user 3 liked it), 33 (user 6 liked it), and 77 (user 2 and user 3 liked it).

  - Note that page 88 is not recommended because user 1 already liked it.



Another example is user 6:

  - User 6 is friends with user 1.

  - User 1 only liked page 88, but user 6 already liked it. Hence, user 6 has no recommendations.



You can recommend pages for users 2, 3, 4, and 5 using a similar process.

解法

SQL