SQL查询查找给定的人，朋友对表中的共同朋友数 [英] SQL Query to find number of mutual friends given a table of person, friend pair

问题描述

我搜索了其他问题，我能够解决部分我想要的东西，但是我无法从中得到进一步的帮助.

I have searched for other questions and I was able to solve part of what I wanted but I couldn't get any further from that.

我在Friends表中有一个包含两列(用户，friend)的表. 下表中将每个用户及其朋友指定为波纹管.

I have a table with two columns(user, friend) in Friends table. Each user and his/her friend is specified in the table as bellow.

User | Friend
1        2
1        6
2        1
2        3
2        6

注意:对于每对(用户，朋友)，都有一行(朋友，用户) 例如:1,2有2,1，因为用户2有朋友1

Note: For every (User, friend) pair there is a row (Friend, User) Eg: 1,2 has 2,1 because User 2 has friend 1

到目前为止，我到达了以下查询，该查询给出了我们指定的配对的共同朋友的数量:

So far, I have arrived at the below query which gives the count of mutual friends for the pair we specify:

select DISTINCT f1.user1 'User', f2.user1 'Friend', COUNT(DISTINCT     f1.user2) 'Mutual friends'
from Friends p
inner join Friends f1 on f1.user2 = p.user1
inner join Friends f2 on f2.user2 = p.user1
where f1.user1 = 2 and f2.user1 = 3 and f1.user2 = f2.user2
group by f1.user1, f2.user1;

我现在的输出:

User    |Friend |Mutual Friends
1            2      1

我想查找整个表格中每对的共同朋友的数量:

I want to find the count of mutual friends of each of pairs throughout the table:

User |  Friend |  Mutual Friends
1        2        1
1        6        0
2        1        1
2        3        0
2        6        0

如何找到所有用户，朋友对的共同朋友的数量?

How do I find the number of mutual friends for all user,friend pairs?

推荐答案

您可以使用自联接:

select f1.user as user1, f2.user as user2, count(*) as num_in_common
from friends f1 join
     friends f2
     on f1.friend = f2.friend 
group by f1.user, f2.user;

如果需要特定用户对的信息，可以添加where子句.

You can add a where clause if you want this information for a particular pair of users.

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