Is this possible with mysql?

Question

First of all: sorry for the title, but maybe I will find a better one later.

I asked this some minutes ago, but since I was not able to describe what I want I try it

Answer 1

     SELECT today.user
          , GROUP_CONCAT(DISTINCT today.ip) ip
       FROM gc_sessions today
       JOIN gc_sessions yesterday
         ON DATE(yesterday.starttime) = DATE(today.starttime) - INTERVAL 1 DAY
        AND today.user = yesterday.user
      WHERE DATE(today.starttime) = '2013-01-10'
      GROUP 
         BY today.user;