select * from
tProblems p 
INNER JOIN 
(
SELECT * FROM tProblemComments order BY uts desc
) as pc ON p.poNumber = pc.poNumber
WHERE open = 1
group by p.poNumber
order by p.poNumber asc

CREATE TABLE `tproblems` (
  `poNumber` bigint(20) NOT NULL default '0',
  `contact` varchar(100) NOT NULL default '',
  `phone` varchar(100) NOT NULL default '',
  `open` tinyint(4) default NULL,
  PRIMARY KEY  (`poNumber`),
  KEY `closed` (`open`)
) 

CREATE TABLE `tproblemcomments` (
  `uts` int(11) NOT NULL default '0',
  `poNumber` int(11) NOT NULL default '0',
  `relevance` varchar(100) NOT NULL default '',
  `comment` text NOT NULL,
  `staff` varchar(20) NOT NULL default '',
  KEY `uts` (`uts`),
  KEY `poNumber` (`poNumber`)
)