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];
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：
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
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