mysql中的order语句_【MySql】1.2 mysql中 Order By 语句的用法
一、用法介绍
Order by语句是用来排序的，默认为升序排序(即：从1到9，从a到z)，order by desc为降序排序(即：从9到1，从z到a)
SELECT column_name(s)
FROM table_name
ORDER BY column_name DESC
例：
mysql> select * from mysql_order_by_test order by uid asc;
+-----+-------+-----+
| uid | name| sex |
+-----+-------+-----+
|1 | ZHANG |1 |
|2 | LI|2 |
|3 | WANG|1 |
+-----+-------+-----+
3 rows in set (0.00sec)
这条语句是按照uid正序查询数据，也就是按照uid从小到大排列
例：
mysql> select * from mysql_order_by_test order by uid desc;
+-----+-------+-----+
| uid | name| sex |
+-----+-------+-----+
|3 | WANG|1 |
|2 | LI|2 |
|1 | ZHANG |1 |
+-----+-------+-----+
3 rows in set (0.00sec)
MySQL Order By根据两列进行分类：
很多时候，我们需要同时根据两列内容(或者更多列)来对数据进行分类。当指定的列数多于一列时，仅在第一列的值完全相同时才参考第二列：
SELECT column_name(s)
FROM table_name
ORDER BY column_name1, column_name2
二、mysql "group by"与"order by"的研究
1.先创建一个表，并添加数据
CREATE TABLE `test` (
`id` INT(10) NOT NULL AUTO_INCREMENT,
`name` VARCHAR(255) NOT NULL,
`category_id` INT(10) NOT NULL,
`date` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP, -- 默认值为当前系统时间
PRIMARY KEY (`id`)
)
ENGINE=MyISAM ROW_FORMAT=DEFAULT;
INSERT INTO `test` (`id`, `name`, `category_id`, `date`) VALUES
(1, 'aaa', 1, '2010-06-10 19:14:37'),
(2, 'bbb', 2, '2010-06-10 19:14:55'),
(3, 'ccc', 1, '2010-06-10 19:16:02'),
(4, 'ddd', 1, '2010-06-10 19:16:15'),
(5, 'eee', 2, '2010-06-10 19:16:35');
现在需要取出每个分类中最新的内容：
(1)如果select * from test group by category_id order by `date`，结果：
明显，这不是想要的数据，原因是只从分组后的每个组里取得了第一条信息，分析如下：
2.根据上面的分析，有两个解决方法：
(1)where+group by(对小组进行排序)
分析分组后的结构：idcategory_id
11
31
41
idcategory_id
22
52
对group by里的小组进行排序的函数我只查到group_concat()可以进行排序，但该函数的作
用是将小组里的字段里的值进行串联起来。
select group_concat(id order by `date` desc) from `test` group by category_id;
select SUBSTRING_INDEX(group_concat(id order by `date` desc),',',1) from `test`
group by category_id
select * from test where id in(select SUBSTRING_INDEX(group_concat(id order by `date` desc),',',1) from `test` group by category_id) order by date desc;
(2)从from返回的数据下手脚(即用子查询)
select * from `test` order by `date` desc
分析分组后的结构：idcategory_id
52
22
idcategory_id
41
31
11
select * from (select * from `test` order by `date` desc) `temp`
group by category_id order by `date` desc
