MySQL全面瓦解9：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：

1、需要排序的字段跟在order by之后；

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

3、排序可以指定多次字段，多字段排序之间用逗号隔开。

4、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;

1、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。

2、count：跳过偏移量offset之后开始取的数据行数，有count行。

3、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

获取(m,n)区间记录

即跳过m条，获取n条，示例如下，我们跳过两条，从第三条开始，连取四条的操作：

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

分页的做法与这个类似，我们程序业务上看到的分页一般有 pageIndex，pageSize等参数，我们通常的做法是 limit pageIndex*pageSize,pageSize。

这边假设有31条数据，每页数量pageSize=10，页面索引pageIndex默认0，则第一页就是 limit 0,10，第二页 limit10,10，第三页 limit 20,10，第四页 limit 30,10。

注意点：

1、limit 后面不能使用表达式，只能使用明确的数值，否则会爆出异常，比如 limit 0*10,10，是不对的，这个上面提过了。

2、limit后续的数值只能是正整数和0，也就是说，不能是负数，否则同样会报错。

3、排序字段的值相同情况下，排序后分页会出现混乱重复的情况。

第3点详细说明下：假如根据age排序，但是有多个age都是20岁的同学，这时候我们3条记录一页，就会出现分页混乱数据重复。因为年龄相同的人有多个，这是几个人的排序在每次查询的时候会有不确定性。

举个例子，下面的分页，混乱了

 mysql> select * from user3;
 +----+------+-------+
 | id | age  | name  |
 +----+------+-------+
 |  1 |   20 | brand |
 |  2 |   22 | sol   |
 |  3 |   20 | helen |
 |  4 | 19.5 | diny  |
 |  6 |   19 | a     |
 |  7 |   20 | b     |
 |  8 |   20 | c     |
 |  9 |   20 | d     |
 | 10 |   20 | e     |
 | 11 |   23 | f     |
 +----+------+-------+
 10 rows in set
 
 mysql> select * from user3 order by age limit 0,3; 
 +----+------+-------+
 | id | age  | name  |
 +----+------+-------+
 |  6 |   19 | a     |
 |  4 | 19.5 | diny  |
 |  3 |   20 | helen |
 +----+------+-------+
 3 rows in set
 
 mysql> select * from user3 order by age limit 3,3;
  
 +----+-----+-------+
 | id | age | name  |
 +----+-----+-------+
 |  3 |  20 | helen |
 |  7 |  20 | b     |
 |  8 |  20 | c     |
 +----+-----+-------+
 3 rows in set
 
 mysql> select * from user3 order by age limit 6,3;
  
 +----+-----+-------+
 | id | age | name  |
 +----+-----+-------+
 |  7 |  20 | b     |
 |  3 |  20 | helen |
 |  2 |  22 | sol   |
 +----+-----+-------+
 3 rows in set 

我们的做法是使用重复值字段做排序的时候再加个唯一依据（一般可以设主键），就不会混乱了。

如下示例，正常了：

 mysql> select * from user3;
 +----+------+-------+
 | id | age  | name  |
 +----+------+-------+
 |  1 |   20 | brand |
 |  2 |   22 | sol   |
 |  3 |   20 | helen |
 |  4 | 19.5 | diny  |
 |  6 |   19 | a     |
 |  7 |   20 | b     |
 |  8 |   20 | c     |
 |  9 |   20 | d     |
 | 10 |   20 | e     |
 | 11 |   23 | f     |
 +----+------+-------+
 10ows in set
17 
 mysql> select * from user3 order by age,idimit 0,3;
19  
 +----+------+-------+
 | id | age  | name  |
 +----+------+-------+
 |  6 |   19 | a     |
 |  4 | 19.5 | diny  |
 |  1 |   20 | brand |
 +----+------+-------+
 3ows in set
28 
 mysql> select * from user3 order by age,idimit 3,3;
30  
 +----+-----+-------+
 | id | age | name  |
 +----+-----+-------+
 |  3 |  20 | helen |
 |  7 |  20 | b     |
 |  8 |  20 | c     |
 +----+-----+-------+
 3ows in set
39 
 mysql> select * from user3 order by age,idimit 6,3;
41  
 +----+-----+------+
 | id | age | name |
 +----+-----+------+
 |  9 |  20 | d    |
 | 10 |  20 | e    |
 |  2 |  22 | sol  |
 +----+-----+------+
 3 rows in set

上述总结
1、order by cname [asc|desc] 用于对查询结果排序，asc为升序，desc为降序，可以省略，省略情况下默认为asc。

2、limit用来限制查询结果返回的行数，包含2个参数（offset，count），offset：表示跳过多少行，count：表示跳过offset行之后取的行数。limit中offset可以省略，默认值为0；limit中offset 和 count都必须大于等于0；limit中offset和count的值不能用表达式。

3、分页排序时，排序字段不要有重复值，重复值情况下可能会导致分页结果混乱，建议在后面加一个主键排序。