MySQL分组然后取每个分组中按照某些字段排序的topN条数据

CREATE TABLE `t` (
 `a` int(11) DEFAULT NULL,
 `b` int(11) DEFAULT NULL,
 `c` int(11) DEFAULT NULL,
 `itime` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO t(`a`, `b`, `c`, `itime`) VALUES ('1', '1', '1', '2014-12-04 19:07:01');
INSERT INTO t(`a`, `b`, `c`, `itime`) VALUES ('1', '1', '2', '2014-12-04 19:07:02');
INSERT INTO t (`a`, `b`, `c`, `itime`) VALUES ('1', '1', '3', '2014-12-04 19:07:03');
INSERT INTO t (`a`, `b`, `c`, `itime`) VALUES ('1', '1', '4', '2014-12-04 19:07:04');
INSERT INTO t(`a`, `b`, `c`, `itime`) VALUES ('1', '2', '1', '2014-12-04 19:07:01');
INSERT INTO t (`a`, `b`, `c`, `itime`) VALUES ('1', '2', '2', '2014-12-04 19:07:02');
INSERT INTO t(`a`, `b`, `c`, `itime`) VALUES ('1', '2', '3', '2014-12-04 19:07:03');
INSERT INTO t (`a`, `b`, `c`, `itime`) VALUES ('1', '2', '4', '2014-12-04 19:07:04');
INSERT INTO t(`a`, `b`, `c`, `itime`) VALUES ('1', '2', '5', '2014-12-04 19:07:05');
INSERT INTO t (`a`, `b`, `c`, `itime`) VALUES ('1', '2', '6', '2014-12-04 19:07:06');

SELECT
	t.a,
 	t.b,
	substring_index(
	group_concat(
	IFNULL(t.c,0)
	ORDER BY
	t.itime DESC
	),
	",",
	3
	) c,
	substring_index(
	group_concat(
	t.itime
	ORDER BY
	t.itime DESC
	),
	",",
	3
	) time
FROM
	t t
GROUP BY
	t.a ,t.b;