title: mysql query help to find mutual friends on social networking site
body: <p>I have a database table called users with a primary key of user_id for each user.</p>
  
  <p>I also have a table called friends with two fields, user_id and friend_user_id. </p>
  
  <p>The user_id field is always the lowest of the two user_id's in order to avoid duplicate entries. </p>
  
  <p>Say I have two users in mind, (lets say user id 1 and user id 4 although they could be anything). </p>
  
  <p>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)?</p>