本节内容：
mysql 分组查询。
一、表结构
--
-- 表的结构 `client_status`
--
CREATE TABLE IF NOT EXISTS `client_status` (
`id` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
`client_id` mediumint(8) unsigned NOT NULL,
`addtime` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`id`),
)
--
-- 转存表中的数据 `client_status`
--
INSERT INTO `client_status` (`id`, `client_id`, `addtime`) VALUES
(3, 1, '2012-03-26 10:32:30'),
(4, 1, '2012-03-26 10:30:31'),
(5, 1, '2012-03-26 10:30:32'),
(6, 2, '2012-03-26 10:30:33'),
(7, 2, '2012-03-26 10:30:34'),
(8, 4, '2012-03-26 10:30:35'),
(9, 4, '2012-03-26 10:30:36'),
(10, 3, '2012-03-26 10:30:37'),
(11, 3, '2012-03-26 10:30:38');
二、需求目的
按client_id分组，查询每个client_id中离目前时间最近的记录。
查询结果示意，如下：
三、解决方法(mysql 分组查询实例)
例1：
SELECT * FROM (SELECT * FROM client_status ORDER BY addtime DESC )a GROUP BY client_id
explain分析
例2：
SELECT * FROM client_status a WHERE NOT EXISTS (SELECT 1 FROM client_status WHERE a.client_id = client_id AND a.addtime < addtime)
explain分析
例3：
SELECT * FROM `client_status` WHERE addtime IN (SELECT max( addtime ) FROM client_status GROUP BY client_id )
explain分析
