SELECT users.uid AS uid, users.name AS username
FROM users
LEFT JOIN user_relationships as ur1 ON ur1.requestee_id = users.uid
LEFT JOIN user_relationships as ur2 on ur2.requestee_id = ur1.requester_id
LEFT JOIN user_relationships as ur3 on ur3.requestee_id = ur2.requestee_id
WHERE
ur2.requester_id = '1'
AND
ur2.requestee_id IS NULL
AND
users.uid != '1'