Question

I have a database table called users with a primary key of user_id for each user.

I also have a table called friends with two fields, user_id and friend_user_id.

The user_id field is always the lowest of the two user_id’s in order to avoid duplicate entries.

Say I have two users in mind, (lets say user id 1 and user id 4 although they could be anything).

How would I return all rows from the users table for users that are friends with user 1 and user 4 (i.e mutual friends)?

Answer 1

I will give you the recipe:

1. Find all friends of user 1
2. Find all friends of user 2
3. Intersect them and the result will be the mutual friends.

Much like this:

UPDATE: Here’s the query:

select f.friend_user_id  from friends f where f.friend_user_id in (
   select friend_user_id from friends where user_id=<id_of_user_a>)
and f.user_id=<id_of_user_b>

The ids returned by above query will be the id of all the users that are mutual friends of user_a and user_b. If you want to get all the details (name, etc) about those users, then do this:

   select f.friend_user_id,u.*  from friends f inner join users u 
   on u.user_id=f.friend_user_id
   where f.friend_user_id in (
   select friend_user_id from friends where user_id=<id_of_user_a>)
   and f.user_id=<id_of_user_b>