MySQL 查询的排序、分页相关

概述

数据库中的数据直接呈现出来一般不是我们想要的，所以我们上两节演示了如何对数据进行过滤的方法。除了对数据进行过滤，

我们可能还需要对数据进行排序，比如想从列表中了解消费最高的项，就可能需要对金额字段做降序排序，想看年龄从小到大的分布情况，就可能需要对user表的age字段进行升序排序。

也可能需要对数据进行限制，比如我们需要对付款的1~10，11~20，21~30 名的用户分别赠予不同的礼品，这时候对数据的限制就很有用了。

备注：下面脚本中[]包含的表示可选，| 分隔符表示可选其一。

数据排序 order by

语法格式如下：

需要排序的字段跟在order by之后；

asc 和 desc表示排序的规则，asc：升序，desc：降序，默认为升序 asc；

排序可以指定多次字段，多字段排序之间用逗号隔开。

多字段排序中，越靠前优先级越高，下面中cname1优先排序，当cname1等值的时候，cname2开始排序，直至所有字段都排序完。

select cname from tname order by cname1 [asc|desc],cname2 [asc|desc]...;

单个字段排序

举个例子，在销售额中通按照交易的订单进行金额额度降序的方式显示：

Mysql> select * from t_order;
+---------+---------+---------+-------+
| orderid | account | amount | Goods |
+---------+---------+---------+-------+
|    8 | brand  | 52.2  |   2 |
|    9 | hen   | 1752.02 |   7 |
|   10 | helyn  | 88.5  |   4 |
|   11 | sol   | 1007.9 |  11 |
|   12 | diny  | 12   |   1 |
|   13 | weng  | 52.2  |   5 |
|   14 | sally  | 99.71  |   9 |
+---------+---------+---------+-------+
7 rows in set

mysql> select * from t_order order by amount desc;
+---------+---------+---------+-------+
| orderid | account | amount | goods |
+---------+---------+---------+-------+
|    9 | hen   | 1752.02 |   7 |
|   11 | sol   | 1007.9 |  11 |
|   14 | sally  | 99.71  |   9 |
|   10 | helyn  | 88.5  |   4 |
|    8 | brand  | 52.2  |   2 |
|   13 | weng  | 52.2  |   5 |
|   12 | diny  | 12   |   1 |
+---------+---------+---------+-------+
7 rows in set

多个字段排序

多个字段排序用逗号隔开，优先级从左到右逐次递减，如下图，如果金额一致，则按照购买商品数量从多到少排序：

mysql> select * from t_order order by amount desc,goods desc;
+---------+---------+---------+-------+
| orderid | account | amount | goods |
+---------+---------+---------+-------+
|    9 | hen   | 1752.02 |   7 |
|   11 | sol   | 1007.9 |  11 |
|   14 | sally  | 99.71  |   9 |
|   10 | helyn  | 88.5  |   4 |
|   13 | weng  | 52.2  |   5 |
|    8 | brand  | 52.2  |   2 |
|   12 | diny  | 12   |   1 |
+---------+---------+---------+-------+
7 rows in set

按alias排序

按照别名排序或者做条件查询的目的都是为了简化代码，方便使用，别名可以是英文，也可以是中文：

mysql> select account as ac,amount as am,goods as gd from t_order order by am,gd desc;

+-------+---------+----+
| ac  | am   | gd |
+-------+---------+----+
| diny | 12   | 1 |
| weng | 52.2  | 5 |
| brand | 52.2  | 2 |
| helyn | 88.5  | 4 |
| sally | 99.71  | 9 |
| sol  | 1007.9 | 11 |
| hen  | 1752.02 | 7 |
+-------+---------+----+
7 rows in set

字段排序中使用函数

下面使用了abs取绝对值函数，所以在 am字段降序排序中，-99.99 排在 99.71之上。

mysql> select * from t_order;
+---------+---------+---------+-------+
| orderid | account | amount | goods |
+---------+---------+---------+-------+
|    8 | brand  | 52.2  |   2 |
|    9 | hen   | 1752.02 |   7 |
|   10 | helyn  | 88.5  |   4 |
|   11 | sol   | 1007.9 |  11 |
|   12 | diny  | 12   |   1 |
|   13 | weng  | 52.2  |   5 |
|   14 | sally  | 99.71  |   9 |
|   15 | brand1 | -99.99 |   5 |
+---------+---------+---------+-------+
8 rows in set

mysql> select account as ac,amount as am,goods as gd from t_order order by abs(am) desc;

+--------+---------+----+
| ac   | am   | gd |
+--------+---------+----+
| hen  | 1752.02 | 7 |
| sol  | 1007.9 | 11 |
| brand1 | -99.99 | 5 |
| sally | 99.71  | 9 |
| helyn | 88.5  | 4 |
| brand | 52.2  | 2 |
| weng  | 52.2  | 5 |
| diny  | 12   | 1 |
+--------+---------+----+
8 rows in set

与Where条件结合使用

order 在 where 条件之后，根据where已经过滤好的数据再进行排序。下面是过滤出购买金额>80 且 购买数量>5的数据，并且按照价格降序排序。

mysql> select * from t_order;
+---------+---------+---------+-------+
| orderid | account | amount | goods |
+---------+---------+---------+-------+
|    8 | brand  | 52.2  |   2 |
|    9 | hen   | 1752.02 |   7 |
|   10 | helyn  | 88.5  |   4 |
|   11 | sol   | 1007.9 |  11 |
|   12 | diny  | 12   |   1 |
|   13 | weng  | 52.2  |   5 |
|   14 | sally  | 99.71  |   9 |
|   15 | brand1 | -99.99 |   5 |
+---------+---------+---------+-------+
8 rows in set

mysql> select * from t_order where amount>80 and goods>5 order by amount desc;
+---------+---------+---------+-------+
| orderid | account | amount | goods |
+---------+---------+---------+-------+
|    9 | hen   | 1752.02 |   7 |
|   11 | sol   | 1007.9 |  11 |
|   14 | sally  | 99.71  |   9 |
+---------+---------+---------+-------+

数据limit

很多时候我们过滤出符合要求的数据之后，还需要得到这些数据中的某一个具体区间，比如对付款超过1000的用户的第1~10，11~20，21~30 名分别赠予不同的礼品，这时候就要使用limit操作了。

limit用来限制select查询返回的数据，常用于数据排行或者分页等情况。

语法格式如下：

select cname from tname limit [offset,] count;

offset表示偏移量，就是指跳过的行数，可以省略不写，默认为0，表示跳过0行，如 limit 8 等同于 limit 0,8。

count：跳过偏移量offset之后开始取的数据行数，有count行。

limit中offset和count的值不能用表达式。

获取前n条记录

如下图，limit n 和 limit 0,n 是一致的：

mysql> select * from t_order;
+---------+---------+---------+-------+
| orderid | account | amount | goods |
+---------+---------+---------+-------+
|    8 | brand  | 52.2  |   2 |
|    9 | hen   | 1752.02 |   7 |
|   10 | helyn  | 88.5  |   4 |
|   11 | sol   | 1007.9 |  11 |
|   12 | diny  | 12   |   1 |
|   13 | weng  | 52.2  |   5 |
|   14 | sally  | 99.71  |   9 |
|   15 | brand1 | -99.99 |   5 |
+---------+---------+---------+-------+
8 rows in set

mysql> select * from t_order limit 2
;
+---------+---------+---------+-------+
| orderid | account | amount | goods |
+---------+---------+---------+-------+
|    8 | brand  | 52.2  |   2 |
|    9 | hen   | 1752.02 |   7 |
+---------+---------+---------+-------+
2 rows in set

mysql> select * from t_order limit 0,2;
+---------+---------+---------+-------+
| orderid | account | amount | goods |
+---------+---------+---------+-------+
|    8 | brand  | 52.2  |   2 |
|    9 | hen   | 1752.02 |   7 |
+---------+---------+---------+-------+
2 rows in set

limit限制单条记录

这边我们获取支付金额中最大和最小的的一条记录。可以先使用 order 条件进行排序，然后limit 第1条记录即可：

 mysql> select * from t_order;
 +---------+---------+---------+-------+
 | orderid | account | amount | goods |
 +---------+---------+---------+-------+
 |    8 | brand  | 52.2  |   2 |
 |    9 | hen   | 1752.02 |   7 |
 |   10 | helyn  | 88.5  |   4 |
 |   11 | sol   | 1007.9 |  11 |
 |   12 | diny  | 12   |   1 |
 |   13 | weng  | 52.2  |   5 |
 |   14 | sally  | 99.71  |   9 |
 |   15 | brand1 | -99.99 |   5 |
 +---------+---------+---------+-------+
 8 rows in set

 mysql> select * from t_order where amount>0 order by amount desc limit 1;
 +---------+---------+---------+-------+
 | orderid | account | amount | goods |
 +---------+---------+---------+-------+
 |    9 | hen   | 1752.02 |   7 |
 +---------+---------+---------+-------+
 1 row in set

 mysql> select * from t_order where amount>0 order by amount asc limit 1;
 +---------+---------+--------+-------+
 | orderid | account | amount | goods |
 +---------+---------+--------+-------+
 |   12 | diny  | 12   |   1 |
 +---------+---------+--------+-------+
 1 row in set

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