Mutual friends sql with join (Mysql)

Question

I have two tables

users table:

id|name

user_relationships

id | user_id | friend_id

and want to ge

Answer 1

You need to join user_relationships with itself, so that two rows with different user_id have the same friend_id

All mutual friends:

select ur1.user_id user1, 
       ur2.user_id user2, 
       ur2.friend_id mutual_friend
from   user_relationships ur1 
       JOIN user_relationships ur2 ON  ur1.friend_id = ur2.friend_id
where  ur1.user_id != ur2.user_id

Join with users table to get the names:

select ur1.user_id user_id1, 
        u1.name User1, 
       ur2.user_id user2, 
        u2.name User2,
       ur2.friend_id mutual_friend_id,
        u3.name mutual_friend
from user_relationships ur1 
     JOIN user_relationships ur2 ON  ur1.friend_id = ur2.friend_id
     JOIN user u1 ON u1.user_id = ur1.user_id
     JOIN user u2 ON u1.user_id = ur2.user_id
     JOIN user u3 ON ur1.user_id = u3.user_id
where ur1.user_id != ur2.user_id

You can filter for mutual friends for some specific users using ur1.user_id = first_user and ur2.user_id = second_user