I made little board, where all postings are in one table:
Field         Type
------------  ---------------
topic_id      int(6) unsigned
topic_pid     int(6) unsigned
author        varchar(50)
mail          varchar(255)
posting_time  timestamp(14)
ip            varchar(15)
body          text
reg_user      tinyint(1)
topic_pid shows what kind of topic is, if 0 then its new thread else reply for exist one.

Now i want to sort threads by last posting_time of replyes. Is it possible to do this without sub query and if yes then how?