I think that the question "select sin(1)" is closer to "select sign(1)" than the answer ".999..." to "0.84..." so i picked the answer "1 for positive"...

Another possible solution:

SELECT * ,

 (SELECT TOP 1 MAILBOX_TYPE 

  FROM user1  

  WHER user1.USER_ID = subscriber.user_id 

  ORDER BY CASE MAILBOX_TYPE WHEN 2 THEN 0 ELSE 1 END) 

FROM subscriber