Leetcode1264. 页面推荐（中等）

题目
朋友关系列表： Friendship

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| user1_id      | int     |
| user2_id      | int     |
+---------------+---------+

这张表的主键是 (user1_id, user2_id)。
这张表的每一行代表着 user1_id 和 user2_id 之间存在着朋友关系。

喜欢列表： Likes

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| user_id     | int     |
| page_id     | int     |
+-------------+---------+

这张表的主键是 (user_id, page_id)。
这张表的每一行代表着 user_id 喜欢 page_id。

写一段 SQL 向user_id = 1 的用户，推荐其朋友们喜欢的页面。不要推荐该用户已经喜欢的页面。

你返回的结果中不应当包含重复项。

返回结果的格式如下例所示：

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:

+------------------+
| recommended_page |
+------------------+
| 23               |
| 24               |
| 56               |
| 33               |
| 77               |
+------------------+

用户1 同 用户2, 3, 4, 6 是朋友关系。
推荐页面为： 页面23 来自于 用户2, 页面24 来自于 用户3, 页面56 来自于 用户3 以及 页面33 来自于 用户6。
页面77 同时被 用户2 和 用户3 推荐。
页面88 没有被推荐，因为 用户1 已经喜欢了它。

解答
先选择user_id = 1 的用户的朋友

select F.user1_id 
from Friendship as F
where F.user2_id =1
union 
select F.user2_id 
from Friendship as F
where F.user1_id =1

选出user_id = 1 的用户喜欢的页面

select L.page_id
from Likes as L
where L.user_id = 1;

选出page_id 不为user_id = 1 的用户喜欢的页面中的user_id为其朋友的页面
注意要进行去重

select distinct LL.page_id as recommended_page
from Likes as LL
where LL.page_id not in (select L.page_id
from Likes as L
where L.user_id = 1) 
and LL.uesr_id in (select F.user1_id 
from Friendship as F
where F.user2_id =1
union 
select F.user2_id 
from Friendship as F
where F.user1_id =1)

后两种写法的想法与上边是一致的

select distinct page_id as recommended_page
from Likes,friendship
where page_id
not in
(select page_id from likes where user_id=1)
and
( user_id in (select user1_id from friendship where user2_id=1)
or
user_id in (select user2_id from friendship where user1_id=1));

SELECT DISTINCT(page_id) AS recommended_page 
FROM Likes 
WHERE (user_id IN (
    SELECT user2_id
    FROM Friendship
    WHERE user1_id = 1 
) OR user_id IN (
    SELECT user1_id
    FROM Friendship
    WHERE user2_id = 1
))
AND page_id NOT IN (
    SELECT page_id AS liked_ids 
    FROM Likes
    WHERE user_id  = 1);