SQL查询查找给定的人，朋友对表的共同朋友数

Question

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.

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

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

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;

The output that I have now:

User    |Friend |Mutual Friends
1            2      1

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

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

Answer 1

You can use a self-join:

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;

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