按某些条件滚动数据库条目

也许你想要的是:

(select max(runstamp) from 
   (select runstamp from mytable order by runstamp limit m)) 

   my $truncate_query = "DELETE FROM mytable WHERE runstamp < " .
       "(SELECT MIN(runstamp) FROM " .
       "(SELECT * FROM mytable ORDER BY runstamp DESC LIMIT $max_records))";