CREATE TABLE IF NOT EXISTS `payments` (
  `uid` INT(11) NOT NULL,
  `amount` FLOAT NOT NULL,
  `pay_date` datetime NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

--
-- Дамп данных таблицы `payments`
--

INSERT INTO `payments` (`uid`, `amount`, `pay_date`) VALUES
(1, 1000, '2012-08-01 12:07:10'),
(1, 1500, '2012-08-01 10:29:29'),
(2, 100, '2012-08-01 00:00:00'),
(2, 50, '2012-08-01 09:15:24');



SELECT p.uid, p.amount
  FROM payments p
  JOIN
    (SELECT uid, MAX(pay_date) AS max_dt
       FROM payments
       GROUP BY uid) sel ON p.uid = sel.uid AND p.pay_date = sel.max_dt



uid | amount
1 | 1000
2 | 50