sql – 从每个组中选择前1行

我有一个表格列出了安装的软件版本：

id  | userid | version | datetime
----+--------+---------+------------------------
111 | 75     | 10075   | 2013-03-12 13:40:58.770
112 | 75     | 10079   | 2013-03-12 13:41:01.583
113 | 78     | 10065   | 2013-03-12 14:18:24.463
114 | 78     | 10079   | 2013-03-12 14:22:20.437
115 | 78     | 10079   | 2013-03-12 14:24:01.830
116 | 78     | 10080   | 2013-03-12 14:24:06.893
117 | 74     | 10080   | 2013-03-12 15:31:42.797
118 | 75     | 10079   | 2013-03-13 07:03:56.157
119 | 75     | 10080   | 2013-03-13 07:05:23.137
120 | 65     | 10080   | 2013-03-13 07:24:33.323
121 | 68     | 10080   | 2013-03-13 08:03:24.247
122 | 71     | 10080   | 2013-03-13 08:20:16.173
123 | 78     | 10080   | 2013-03-13 08:28:25.487
124 | 56     | 10080   | 2013-03-13 08:49:44.503

我想显示每个用户名的一个记录的所有字段,但只显示最高版本(也是版本是varchar).

解决方法

你没有指定你想要如何处理关系,但是如果你想要重复的显示;

SELECT a.* FROM MyTable a
LEFT JOIN MyTable b
  ON a.userid=b.userid
 AND CAST(a.version AS INT) < CAST(b.version AS INT)
WHERE b.version IS NULL

An SQLfiddle to test with.

如果你想删除重复的东西,如果他们存在选择最新的,你必须稍微扩展查询;

WITH cte AS (SELECT *,CAST(version AS INT) num_version FROM MyTable)
SELECT a.id,a.userid,a.version,a.datetime 
FROM cte a LEFT JOIN cte b
  ON a.userid=b.userid
 AND (a.num_version < b.num_version OR 
     (a.num_version = b.num_version AND a.[datetime]<b.[datetime]))
WHERE b.version IS NULL

Another SQLfiddle.