Mysql排序和分页(order by&limit)及存在的坑

排序查询（order by）

电商中：我们想查看今天所有成交的订单，按照交易额从高到低排序，此时我们可以使用数据库中的排序功能来完成。

排序语法：

select 字段名 from 表名 order by 字段1 [asc|desc],字段2 [asc|desc]; 需要排序的字段跟在order by之后； asc|desc表示排序的规则，asc：升序，desc：降序，默认为asc； 支持多个字段进行排序，多字段排序之间用逗号隔开。

单字段排序

mysql> create table test2(a int,b varchar(10));Query OK, 0 rows affected (0.01 sec)mysql> insert into test2 values (10,’jack’),(8,’tom’),(5,’ready’),(100,’javacode’);Query OK, 4 rows affected (0.00 sec)Records: 4 Duplicates: 0 Warnings: 0mysql> select * from test2;+------+----------+| a | b |+------+----------+| 10 | jack || 8 | tom || 5 | ready || 100 | javacode |+------+----------+4 rows in set (0.00 sec)mysql> select * from test2 order by a asc;+------+----------+| a | b |+------+----------+| 5 | ready || 8 | tom || 10 | jack || 100 | javacode |+------+----------+4 rows in set (0.00 sec)mysql> select * from test2 order by a desc;+------+----------+| a | b |+------+----------+| 100 | javacode || 10 | jack || 8 | tom || 5 | ready |+------+----------+4 rows in set (0.00 sec)mysql> select * from test2 order by a;+------+----------+| a | b |+------+----------+| 5 | ready || 8 | tom || 10 | jack || 100 | javacode |+------+----------+4 rows in set (0.00 sec)

多字段排序

比如学生表，先按学生年龄降序，年龄相同时，再按学号升序，如下：

mysql> create table stu(id int not null comment ’学号’ primary key,age tinyint not null comment ’年龄’,name varchar(16) comment ’姓名’);Query OK, 0 rows affected (0.01 sec)mysql> insert into stu (id,age,name) values (1001,18,’路人甲Java’),(1005,20,’刘德华’),(1003,18,’张学友’),(1004,20,’张国荣’),(1010,19,’梁朝伟’);Query OK, 5 rows affected (0.00 sec)Records: 5 Duplicates: 0 Warnings: 0mysql> select * from stu;+------+-----+---------------+| id | age | name |+------+-----+---------------+| 1001 | 18 | 路人甲Java || 1003 | 18 | 张学友 || 1004 | 20 | 张国荣 || 1005 | 20 | 刘德华 || 1010 | 19 | 梁朝伟 |+------+-----+---------------+5 rows in set (0.00 sec)mysql> select * from stu order by age desc,id asc;+------+-----+---------------+| id | age | name |+------+-----+---------------+| 1004 | 20 | 张国荣 || 1005 | 20 | 刘德华 || 1010 | 19 | 梁朝伟 || 1001 | 18 | 路人甲Java || 1003 | 18 | 张学友 |+------+-----+---------------+5 rows in set (0.00 sec)

按别名排序

mysql> select * from stu;+------+-----+---------------+| id | age | name |+------+-----+---------------+| 1001 | 18 | 路人甲Java || 1003 | 18 | 张学友 || 1004 | 20 | 张国荣 || 1005 | 20 | 刘德华 || 1010 | 19 | 梁朝伟 |+------+-----+---------------+5 rows in set (0.00 sec)mysql> select age ’年龄’,id as ’学号’ from stu order by 年龄 asc,学号 desc;+--------+--------+| 年龄 | 学号 |+--------+--------+| 18 | 1003 || 18 | 1001 || 19 | 1010 || 20 | 1005 || 20 | 1004 |+--------+--------+

按函数排序

有学生表（id：编号，birth：出生日期，name：姓名），如下：

mysql> drop table if exists student;Query OK, 0 rows affected (0.01 sec)mysql> CREATE TABLE student ( -> id int(11) NOT NULL COMMENT ’学号’, -> birth date NOT NULL COMMENT ’出生日期’, -> name varchar(16) DEFAULT NULL COMMENT ’姓名’, -> PRIMARY KEY (id) -> );Query OK, 0 rows affected (0.01 sec)mysql> insert into student (id,birth,name) values (1001,’1990-10-10’,’路人甲Java’),(1005,’1960-03-01’,’刘德华’),(1003,’1960-08-16’,’张学友’),(1004,’1968-07-01’,’张国荣’),(1010,’1962-05-16’,’梁朝伟’);Query OK, 5 rows affected (0.00 sec)Records: 5 Duplicates: 0 Warnings: 0mysql>mysql> SELECT * FROM student;+------+------------+---------------+| id | birth | name |+------+------------+---------------+| 1001 | 1990-10-10 | 路人甲Java || 1003 | 1960-08-16 | 张学友 || 1004 | 1968-07-01 | 张国荣 || 1005 | 1960-03-01 | 刘德华 || 1010 | 1962-05-16 | 梁朝伟 |+------+------------+---------------+5 rows in set (0.00 sec)

需求：按照出生年份升序、编号升序，查询出编号、出生日期、出生年份、姓名，2种写法如下：

mysql> SELECT id 编号,birth 出生日期,year(birth) 出生年份,name 姓名 from student ORDER BY year(birth) asc,id asc;+--------+--------------+--------------+---------------+| 编号 | 出生日期 | 出生年份 | 姓名 |+--------+--------------+--------------+---------------+| 1003 | 1960-08-16 | 1960 | 张学友 || 1005 | 1960-03-01 | 1960 | 刘德华 || 1010 | 1962-05-16 | 1962 | 梁朝伟 || 1004 | 1968-07-01 | 1968 | 张国荣 || 1001 | 1990-10-10 | 1990 | 路人甲Java |+--------+--------------+--------------+---------------+5 rows in set (0.00 sec)mysql> SELECT id 编号,birth 出生日期,year(birth) 出生年份,name 姓名 from student ORDER BY 出生年份 asc,id asc;+--------+--------------+--------------+---------------+| 编号 | 出生日期 | 出生年份 | 姓名 |+--------+--------------+--------------+---------------+| 1003 | 1960-08-16 | 1960 | 张学友 || 1005 | 1960-03-01 | 1960 | 刘德华 || 1010 | 1962-05-16 | 1962 | 梁朝伟 || 1004 | 1968-07-01 | 1968 | 张国荣 || 1001 | 1990-10-10 | 1990 | 路人甲Java |+--------+--------------+--------------+---------------+5 rows in set (0.00 sec)

说明：year函数：属于日期函数，可以获取对应日期中的年份。上面使用了2种方式排序，第一种是在order by中使用了函数，第二种是使用了别名排序。

where之后进行排序

有订单数据如下：

mysql> drop table if exists t_order;Query OK, 0 rows affected, 1 warning (0.00 sec)mysql> create table t_order( -> id int not null auto_increment comment ’订单编号’, -> price decimal(10,2) not null default 0 comment ’订单金额’, -> primary key(id) -> )comment ’订单表’;Query OK, 0 rows affected (0.01 sec)mysql> insert into t_order (price) values (88.95),(100.68),(500),(300),(20.88),(200.5);Query OK, 6 rows affected (0.00 sec)Records: 6 Duplicates: 0 Warnings: 0mysql> select * from t_order;+----+--------+| id | price |+----+--------+| 1 | 88.95 || 2 | 100.68 || 3 | 500.00 || 4 | 300.00 || 5 | 20.88 || 6 | 200.50 |+----+--------+6 rows in set (0.00 sec)

需求：查询订单金额>=100的，按照订单金额降序排序，显示2列数据，列头：订单编号、订单金额，如下：

mysql> select a.id 订单编号,a.price 订单金额 from t_order a where a.price>=100 order by a.price desc;+--------------+--------------+| 订单编号 | 订单金额 |+--------------+--------------+| 3 | 500.00 || 4 | 300.00 || 6 | 200.50 || 2 | 100.68 |+--------------+--------------+4 rows in set (0.00 sec)

limit介绍

limit用来限制select查询返回的行数，常用于分页等操作。

语法：

select 列 from 表 limit [offset,] count;

说明：

offset：表示偏移量，通俗点讲就是跳过多少行，offset可以省略，默认为0，表示跳过0行；范围：[0,+∞)。 count：跳过offset行之后开始取数据，取count行记录；范围：[0,+∞)。 limit中offset和count的值不能用表达式。

下面我们列一些常用的示例来加深理解。

获取前n行记录

select 列 from 表 limit 0,n;或者select 列 from 表 limit n;

示例，获取订单的前2条记录，如下：

mysql> create table t_order( -> id int not null auto_increment comment ’订单编号’, -> price decimal(10,2) not null default 0 comment ’订单金额’, -> primary key(id) -> )comment ’订单表’;Query OK, 0 rows affected (0.01 sec)mysql> insert into t_order (price) values (88.95),(100.68),(500),(300),(20.88),(200.5);Query OK, 6 rows affected (0.01 sec)Records: 6 Duplicates: 0 Warnings: 0mysql> select * from t_order;+----+--------+| id | price |+----+--------+| 1 | 88.95 || 2 | 100.68 || 3 | 500.00 || 4 | 300.00 || 5 | 20.88 || 6 | 200.50 |+----+--------+6 rows in set (0.00 sec)mysql> select a.id 订单编号,a.price 订单金额 from t_order a limit 2;+--------------+--------------+| 订单编号 | 订单金额 |+--------------+--------------+| 1 | 88.95 || 2 | 100.68 |+--------------+--------------+2 rows in set (0.00 sec)mysql> select a.id 订单编号,a.price 订单金额 from t_order a limit 0,2;+--------------+--------------+| 订单编号 | 订单金额 |+--------------+--------------+| 1 | 88.95 || 2 | 100.68 |+--------------+--------------+2 rows in set (0.00 sec)

获取最大的一条记录

我们需要获取订单金额最大的一条记录，可以这么做：先按照金额降序，然后取第一条记录，如下：

mysql> select a.id 订单编号,a.price 订单金额 from t_order a order by a.price desc;+--------------+--------------+| 订单编号 | 订单金额 |+--------------+--------------+| 3 | 500.00 || 4 | 300.00 || 6 | 200.50 || 2 | 100.68 || 1 | 88.95 || 5 | 20.88 |+--------------+--------------+6 rows in set (0.00 sec)mysql> select a.id 订单编号,a.price 订单金额 from t_order a order by a.price desc limit 1;+--------------+--------------+| 订单编号 | 订单金额 |+--------------+--------------+| 3 | 500.00 |+--------------+--------------+1 row in set (0.00 sec)mysql> select a.id 订单编号,a.price 订单金额 from t_order a order by a.price desc limit 0,1;+--------------+--------------+| 订单编号 | 订单金额 |+--------------+--------------+| 3 | 500.00 |+--------------+--------------+1 row in set (0.00 sec)

获取排名第n到m的记录

我们需要先跳过n-1条记录，然后取m-n+1条记录，如下：

select 列 from 表 limit n-1,m-n+1;

如：我们想获取订单金额最高的3到5名的记录，我们需要跳过2条，然后获取3条记录，如下：

mysql> select a.id 订单编号,a.price 订单金额 from t_order a order by a.price desc;+--------------+--------------+| 订单编号 | 订单金额 |+--------------+--------------+| 3 | 500.00 || 4 | 300.00 || 6 | 200.50 || 2 | 100.68 || 1 | 88.95 || 5 | 20.88 |+--------------+--------------+6 rows in set (0.00 sec)mysql> select a.id 订单编号,a.price 订单金额 from t_order a order by a.price desc limit 2,3;+--------------+--------------+| 订单编号 | 订单金额 |+--------------+--------------+| 6 | 200.50 || 2 | 100.68 || 1 | 88.95 |+--------------+--------------+3 rows in set (0.00 sec)

分页查询

开发过程中，分页我们经常使用，分页一般有2个参数：page：表示第几页，从1开始，范围[1,+∞)pageSize：每页显示多少条记录，范围[1,+∞)如：page = 2，pageSize = 10，表示获取第2页10条数据。我们使用limit实现分页，语法如下：

select 列 from 表名 limit (page - 1) * pageSize,pageSize;

需求：我们按照订单金额降序，每页显示2条，依次获取所有订单数据、第1页、第2页、第3页数据，如下：

mysql> select a.id 订单编号,a.price 订单金额 from t_order a order by a.price desc;+--------------+--------------+| 订单编号 | 订单金额 |+--------------+--------------+| 3 | 500.00 || 4 | 300.00 || 6 | 200.50 || 2 | 100.68 || 1 | 88.95 || 5 | 20.88 |+--------------+--------------+6 rows in set (0.00 sec)mysql> select a.id 订单编号,a.price 订单金额 from t_order a order by a.price desc limit 0,2;+--------------+--------------+| 订单编号 | 订单金额 |+--------------+--------------+| 3 | 500.00 || 4 | 300.00 |+--------------+--------------+2 rows in set (0.00 sec)mysql> select a.id 订单编号,a.price 订单金额 from t_order a order by a.price desc limit 2,2;+--------------+--------------+| 订单编号 | 订单金额 |+--------------+--------------+| 6 | 200.50 || 2 | 100.68 |+--------------+--------------+2 rows in set (0.00 sec)mysql> select a.id 订单编号,a.price 订单金额 from t_order a order by a.price desc limit 4,2;+--------------+--------------+| 订单编号 | 订单金额 |+--------------+--------------+| 1 | 88.95 || 5 | 20.88 |+--------------+--------------+2 rows in set (0.00 sec)

避免踩坑

limit中不能使用表达式

mysql> select * from t_order where limit 1,4+1;ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ’limit 1,4+1’ at line 1mysql> select * from t_order where limit 1+0;ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ’limit 1+0’ at line 1mysql>

结论：limit后面只能够跟明确的数字。

limit后面的2个数字不能为负数

mysql> select * from t_order where limit -1;ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ’limit -1’ at line 1mysql> select * from t_order where limit 0,-1;ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ’limit 0,-1’ at line 1mysql> select * from t_order where limit -1,-1;ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ’limit -1,-1’ at line 1

排序分页存在的坑

准备数据：

mysql> insert into test1 (b) values (1),(2),(3),(4),(2),(2),(2),(2);Query OK, 8 rows affected (0.01 sec)Records: 8 Duplicates: 0 Warnings: 0mysql> select * from test1;+---+---+| a | b |+---+---+| 1 | 1 || 2 | 2 || 3 | 3 || 4 | 4 || 5 | 2 || 6 | 2 || 7 | 2 || 8 | 2 |+---+---+8 rows in set (0.00 sec)mysql> select * from test1 order by b asc;+---+---+| a | b |+---+---+| 1 | 1 || 2 | 2 || 5 | 2 || 6 | 2 || 7 | 2 || 8 | 2 || 3 | 3 || 4 | 4 |+---+---+8 rows in set (0.00 sec)

下面我们按照b升序，每页2条数据，来获取数据。

下面的sql依次为第1页、第2页、第3页、第4页、第5页的数据，如下：

mysql> select * from test1 order by b asc limit 0,2;+---+---+| a | b |+---+---+| 1 | 1 || 2 | 2 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc limit 2,2;+---+---+| a | b |+---+---+| 8 | 2 || 6 | 2 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc limit 4,2;+---+---+| a | b |+---+---+| 6 | 2 || 7 | 2 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc limit 6,2;+---+---+| a | b |+---+---+| 3 | 3 || 4 | 4 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc limit 7,2;+---+---+| a | b |+---+---+| 4 | 4 |+---+---+1 row in set (0.00 sec)

上面有2个问题：

问题1：看一下第2个sql和第3个sql，分别是第2页和第3页的数据，结果出现了相同的数据，是不是懵逼了。

问题2：整个表只有8条记录，怎么会出现第5页的数据呢，又懵逼了。

我们来分析一下上面的原因：主要是b字段存在相同的值，当排序过程中存在相同的值时，没有其他排序规则时，mysql懵逼了，不知道怎么排序了。

就像我们上学站队一样，按照身高排序，那身高一样的时候如何排序呢？身高一样的就乱排了。

建议：排序中存在相同的值时，需要再指定一个排序规则，通过这种排序规则不存在二义性，比如上面可以再加上a降序，如下：

mysql> select * from test1 order by b asc,a desc;+---+---+| a | b |+---+---+| 1 | 1 || 8 | 2 || 7 | 2 || 6 | 2 || 5 | 2 || 2 | 2 || 3 | 3 || 4 | 4 |+---+---+8 rows in set (0.00 sec)mysql> select * from test1 order by b asc,a desc limit 0,2;+---+---+| a | b |+---+---+| 1 | 1 || 8 | 2 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc,a desc limit 2,2;+---+---+| a | b |+---+---+| 7 | 2 || 6 | 2 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc,a desc limit 4,2;+---+---+| a | b |+---+---+| 5 | 2 || 2 | 2 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc,a desc limit 6,2;+---+---+| a | b |+---+---+| 3 | 3 || 4 | 4 |+---+---+2 rows in set (0.00 sec)mysql> select * from test1 order by b asc,a desc limit 8,2;Empty set (0.00 sec)

看上面的结果，分页数据都正常了，第5页也没有数据了。

总结

order by … [asc|desc]用于对查询结果排序，asc：升序，desc：降序，asc|desc可以省略，默认为asc limit用来限制查询结果返回的行数，有2个参数（offset，count），offset：表示跳过多少行，count：表示跳过offset行之后取count行 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 分页排序时，排序不要有二义性，二义性情况下可能会导致分页结果乱序，可以在后面追加一个主键排序

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