特殊sql语句

1.

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

id	client_id	addtime
3	1	2012-03-26 10:32:30
7	2	2012-03-26 10:30:34
11	3	2012-03-26 10:30:38
9	4	2012-03-26 10:30:36

SELECT * FROM (SELECT * FROM client_status ORDER BY addtime DESC )a GROUP BY client_id

判断name字段是否有重复数据：select count(*) num,name FROM user group by name having num>1