Fetch unread messages, by user

SELECT id, message
FROM messages
LEFT JOIN messages_read
    ON messages_read.message_id = messages.id
    AND messages_read.[user_id] = @user_id
WHERE
    messages_read.message_id IS NULL

SELECT id FROM messages m WHERE m.id NOT IN(
    SELECT message_id FROM messages_read WHERE user_id = ?)

SELECT id, body FROM messages LEFT JOIN
  (SELECT message_id FROM messages_read WHERE user_id = ?)
  ON id=message_id WHERE message_id IS NULL