Mutual friends of two users

Table friendships:

A row means the two users are friends. Each friendship is stored once, in either direction, though the same pair can appear in both directions by accident. Friendship is mutual, so (1, 3) makes 3 a friend of 1 and 1 a friend of 3.

Return the mutual friends of users 1 and 2: users who are friends with both.

Output column: friend_id.

CREATE TABLE friendships (
  user_id int NOT NULL,
  friend_id int NOT NULL
);