排序和分页（order by、limit）
排序和分页（order by、limit）
排序查询（order by）
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 test14(a int,b varchar(10));
Query OK, 0 rows affected (0.01 sec)
mysql> insert into test14 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 test14;
+------+----------+
| a | b |
+------+----------+
| 10 | jack |
| 8 | tom |
| 5 | ready |
| 100 | javacode |
+------+----------+
4 rows in set (0.00 sec)
mysql> select * from test14 order by a;
+------+----------+
| a | b |
+------+----------+
| 5 | ready |
| 8 | tom |
| 10 | jack |
| 100 | javacode |
+------+----------+
4 rows in set (0.00 sec)
mysql> select * from test14 order by a asc;
+------+----------+
| a | b |
+------+----------+
| 5 | ready |
| 8 | tom |
| 10 | jack |
| 100 | javacode |
+------+----------+
4 rows in set (0.00 sec)
mysql> select * from test14 order by a desc;
+------+----------+
| a | b |
+------+----------+
| 100 | javacode |
| 10 | jack |
| 8 | tom |
| 5 | ready |
+------+----------+
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 student(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 (0.30 sec)
mysql> insert into student (id,age,name) values (1001,18,'周星驰'),(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 student;
+------+-----+-----------+
| id | age | name |
+------+-----+-----------+
| 1001 | 18 | 周星驰 |
| 1003 | 18 | 张学友 |
| 1004 | 20 | 张国荣 |
| 1005 | 20 | 刘德华 |
| 1010 | 19 | 梁朝伟 |
+------+-----+-----------+
5 rows in set (0.01 sec)
mysql> select * from student 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 | 周星驰 |
| 1003 | 18 | 张学友 |
+------+-----+-----------+
5 rows in set (0.00 sec)
按别名排序
mysql> select * from student;
+------+-----+-----------+
| id | age | name |
+------+-----+-----------+
| 1001 | 18 | 周星驰 |
| 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 student order by `年龄` asc,`学号` desc;
+--------+--------+
| 年龄 | 学号 |
+--------+--------+
| 18 | 1003 |
| 18 | 1001 |
| 19 | 1010 |
| 20 | 1005 |
| 20 | 1004 |
+--------+--------+
5 rows in set (0.00 sec)
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 (0.00 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 (0.00 sec)
mysql> insert into student (id,birth,name) values (1001,'1990-10-10','周星驰'),(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');
mysql> select * from student;
+------+------------+-----------+
| id | birth | name |
+------+------------+-----------+
| 1001 | 1990-10-10 | 周星驰 |
| 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 (0.01 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：
#用year函数取出年份
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 | 周星驰 |
+--------+--------------+--------------+-----------+
5 rows in set (0.05 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 | 周星驰 |
+--------+--------------+--------------+-----------+
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之后排序
某个订单表数据如下：
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 (0.28 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.pricee `订单金额` from _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)
mysql> select a.id `订单编号`,a.price `订单金额` from t_order a where price>=100 order by 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
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
示例：
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 (0.28 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)
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 1,2;
+--------------+--------------+
| 订单编号 | 订单金额 |
+--------------+--------------+
| 2 | 100.68 |
| 3 | 500.00 |
+--------------+--------------+
2 rows in set (0.00 sec)
获取最大或者最小的一条记录
先进行升序或者降序排序，然后去第一条记录
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 limit 1; +--------------+--------------+
| 订单编号 | 订单金额 |
+--------------+--------------+
| 5 | 20.88 |
+--------------+--------------+
1 row in set (0.00 sec)
获取第n到第m的记录
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
在开发过程中我们经常会使用分页，核心技术是使用limit进行数据的读取。
不同数据库分页的方法不同，mysql使用limit
分页一般有2个参数：
page：表示是当前第几页，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。
使用limit实现分页，语法如下：
select 列 from 表名 limit (page - 1) * pageSize,pageSize;
比如：
查询第1条到第10条的数据的sql是：select * from table limit 0,10; ->对应我们的需求就是查询第一页的数据：select * from table limit (1-1)*10,10;
查询第10条到第20条的数据的sql是：select * from table limit 10,20; ->对应我们的需求就是查询第二页的数据：select * from table limit (2-1)*10,10;
查询第20条到第30条的数据的sql是：select * from table limit 20,30; ->对应我们的需求就是查询第三页的数据：select * from table limit (3-1)*10,10;
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)
注意：
-
limit后面不能使用表达式
-
limit后面不能跟负数，因为范围为[0,+∞)
分页排序数据重复问题
mysql> insert into test1 (b) values (1),(2),(3),(4),(2),(2),(2),(2);
Query OK, 8 rows affected (0.02 sec)
Records: 8 Duplicates: 0 Warnings: 0
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)
mysql>
mysql>
mysql>
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)
解决方案：需要增加唯一索引的排序（比如主键）
例如：
mysql> select * from test1 order by b asc,a 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,a asc limit 2,2;
+---+---+
| a | b |
+---+---+
| 5 | 2 |
| 6 | 2 |
+---+---+
2 rows in set (0.00 sec)
mysql> select * from test1 order by b asc,a asc limit 4,2;
+---+---+
| a | b |
+---+---+
| 7 | 2 |
| 8 | 2 |
+---+---+
2 rows in set (0.00 sec)
mysql> select * from test1 order by b asc,a asc limit 6,2;
+---+---+
| a | b |
+---+---+
| 3 | 3 |
| 4 | 4 |
+---+---+
2 rows in set (0.00 sec)
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
