Mysql排序和分页(order by&limit)及存在的坑

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: 0
mysql> 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: 0
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 * 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: 0
mysql>
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: 0
mysql> 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: 0
mysql> 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 1
mysql> 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 1
mysql>

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 1
mysql> 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 1
mysql> 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: 0
mysql> 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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原文链接:https://blog.csdn.net/m0_47157676/article/details/108577231