1 简述
- 本文以MYSQL数据库为例，讲述下数据库的基础操作之增删改查
2 MYSQL数据库安装
-
sudo apt-get install mysql-server mysql-client - 数据库编程还需要安装开发包
-
sudo apt-get install libmysqlclient-dev
3 MYSQL数据库登录
- 安装成功后使用以下命令登录
-
mysql -h [主机名或ip] -u [用户] -p - eg
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mysql -u root -p - 登录成功后，就进入了以下界面
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root@ubuntu:/home/lng# mysql -u root -p Enter password: Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 17 Server version: 8.0.26-0ubuntu0.20.04.2 (Ubuntu) Copyright (c) 2000, 2021, Oracle and/or its affiliates. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners. Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. mysql> - 退出
- exit 或者 quit
4 库的操作
- 库就是仓库，主要存放数据表。
- 库的操作比较简单，主要是以下四种：
4.1 显示库信息
- show databases
- 该命令主要查看数据库中所有的库信息
- eg
-
mysql> show databases; +--------------------+ | Database | +--------------------+ | information_schema | | mysql | | performance_schema | | sys | +--------------------+ 4 rows in set (0.00 sec)
4.2 创建库
- create database 库名
- eg
-
mysql> create database csdn; Query OK, 1 row affected (0.01 sec) mysql> show databases; +--------------------+ | Database | +--------------------+ | csdn | | information_schema | | mysql | | performance_schema | | sys | +--------------------+ 5 rows in set (0.00 sec) mysql> - 创建成功后，我们再查看，就会看到多了一个csdn的库
4.3 删除库
- drop database 库名
- eg
-
mysql> drop database csdn; Query OK, 0 rows affected (0.01 sec) mysql> show databases; +--------------------+ | Database | +--------------------+ | information_schema | | mysql | | performance_schema | | sys | +--------------------+ 4 rows in set (0.00 sec) mysql> - 删除库后，再查看，刚才创建的库就被删除了
- 删库时一定要谨慎操作，库一旦被删除，库下面所有的数据表也会被删除掉。
4.4 选择库
- use 库名
- 如果要对库下面的表进行操作，就要先选择库
-
mysql> show databases; +--------------------+ | Database | +--------------------+ | information_schema | | mysql | | performance_schema | | sys | +--------------------+ 4 rows in set (0.00 sec) mysql> use mysql; Reading table information for completion of table and column names You can turn off this feature to get a quicker startup with -A Database changed mysql>
5 表的操作
5.1 数据类型
-
介绍数据表的操作之前先介绍下mysql数据库的数据类型
-
数值型
类型 字节 说明 TINYINT 1 整数型 SAMLLINT 2 整数型 INT 4 整数型 BIGINT 8 整数型 FLOAT 4 浮点型 DOUBLE 8 浮点型 -
字符串型
类型 字节 说明 CHAR 0 ~ 255 定长字符串 VARCHAR 0 ~ 255 不定长字符串 -
时间日期型
类型 字节 范围 格式 说明 DATA 3 1000-01-01/9999-12-31 YYYY-MM-DD 日期型 TIME 3 -838:59:59/838:59:59 HH:MM:SS 时间值或持续时间 YEAR 1 1901/2155 YYYY 年份值 DATETIME 8 1000-01-01 00:00:00/9999-12-21 23:59:59 YYYY-MM-DDHH:MM:SS 日期和时间值
5.2 约束
-
约束是对插入数据库中的数据进行限定，这么做的目的是为了保证数据的有效性和完整性。
约束关键字 含义 NOT NULL 约束字段值不能为空 DEFAULT 设置字段默认值 UNIQUE KEY 设置字段的值是唯一的 PRIMARY KEY 设置字段为表的主键 AUTO_INCREMENT 设置字段值为自动增加 FOREIGN KEY 设置字段为表的外键
5.3 创建表
-
create table 表名(字段名1 数据类型 约束条件，字段名2 数据类型 约束条件，...) - eg
-
mysql> create database CSDN; Query OK, 1 row affected (0.01 sec) mysql> show databases; +--------------------+ | Database | +--------------------+ | CSDN | | information_schema | | mysql | | performance_schema | | sys | +--------------------+ 5 rows in set (0.00 sec) mysql> use CSDN; Database changed mysql> create table student(id INT PRIMARY KEY,name VARCHAR(30),age INT,score FLOAT); Query OK, 0 rows affected (0.02 sec) - 创建表前，先要选择库，如果没有，可以先创建一个库。表创建好后，可以使用desc命令查看表结构。
5.4 查看表结构
-
describe/desc 表名 - eg
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mysql> desc student; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int | NO | PRI | NULL | | | name | varchar(30) | YES | | NULL | | | age | int | YES | | NULL | | | score | float | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 4 rows in set (0.01 sec) mysql> - 创建好表后，可以查看表的结构。可以看到表的字段名，对应字段的数据类型以及约束等信息
5.5 删除表
-
drop table 表名
5.6 修改表
5.6.1 修改表名
-
alter table 旧表名 rename 新表名
5.6.2 增加字段
- 在表的最后一个位置增加字段
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alter table 表名 add 字段名 字段数据类型 - eg
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mysql> alter table students add class INT; Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc students; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int | NO | PRI | NULL | | | name | varchar(30) | YES | | NULL | | | age | int | YES | | NULL | | | score | float | YES | | NULL | | | class | int | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 5 rows in set (0.01 sec) mysql> - 在最后一个位置插入一个class字段
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- 在表的第一个位置增加字段
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alter table 表名 add 字段名 字段数据类型 first - eg
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mysql> alter table students add code INT first; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc students; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | code | int | YES | | NULL | | | id | int | NO | PRI | NULL | | | name | varchar(30) | YES | | NULL | | | age | int | YES | | NULL | | | score | float | YES | | NULL | | | class | int | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 6 rows in set (0.00 sec) mysql> - 将code字段添加到表的第一个位置
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- 在表的指定位置增加字段
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alter table 表名 add 字段名 字段数据类型 after 字段名 - eg
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mysql> alter table students add sex VARCHAR(10) after name; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc students; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | code | int | YES | | NULL | | | id | int | NO | PRI | NULL | | | name | varchar(30) | YES | | NULL | | | sex | varchar(10) | YES | | NULL | | | age | int | YES | | NULL | | | score | float | YES | | NULL | | | class | int | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 7 rows in set (0.01 sec) mysql> - 我们可以在name字段后面加一个sex字段
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5.6.3 删除字段
-
alter table 表名 drop 字段名
5.6.4 修改字段
- 修改字段的数据类型
-
alter table 表名 modify 字段名 字段新的数据类型 - eg
-
mysql> desc students; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | code | int | YES | | NULL | | | id | int | NO | PRI | NULL | | | name | varchar(30) | YES | | NULL | | | sex | varchar(10) | YES | | NULL | | | age | int | YES | | NULL | | | score | float | YES | | NULL | | | class | int | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 7 rows in set (0.01 sec) mysql> alter table students modify score DOUBLE; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc students; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | code | int | YES | | NULL | | | id | int | NO | PRI | NULL | | | name | varchar(30) | YES | | NULL | | | sex | varchar(10) | YES | | NULL | | | age | int | YES | | NULL | | | score | double | YES | | NULL | | | class | int | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 7 rows in set (0.00 sec) mysql> - 可以将score字段的数据类型由float改为double
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- 修改字段名称
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alter table 表名 change 旧字段名 新字段名 旧字段类型 - eg
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mysql> desc students; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | code | int | YES | | NULL | | | id | int | NO | PRI | NULL | | | name | varchar(30) | YES | | NULL | | | sex | varchar(10) | YES | | NULL | | | age | int | YES | | NULL | | | score | double | YES | | NULL | | | class | int | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 7 rows in set (0.00 sec) mysql> alter table students change code lcode INT; Query OK, 0 rows affected (0.00 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc students; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | lcode | int | YES | | NULL | | | id | int | NO | PRI | NULL | | | name | varchar(30) | YES | | NULL | | | sex | varchar(10) | YES | | NULL | | | age | int | YES | | NULL | | | score | double | YES | | NULL | | | class | int | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 7 rows in set (0.00 sec) mysql> - 将code字段名称修改为了lcode
-
- 修改字段名称和类型
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alter table 表名 change 旧字段名 新字段名 新字段类型
-
- 修改字段顺序
- 将字段放到第一个位置
-
alter table 表名 modify 字段名 字段类型 first - 将字段放到指定位置
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alter table 表名 modify 字段名1 字段类型 after 字段名2
- 约束设置
- 待完成
6 数据的操作
- 前面介绍了表的一些操作，接下来就主要介绍下数据的增删改查
6.1 插入数据
- 单条数据插入
-
insert into 表名(字段1, 字段2, 字段3, ...) values(值1, 值2, 值3, ...) - eg
-
mysql> insert into students(id,name,age,score) values(1001,'TOM',22,90.5); Query OK, 1 row affected (0.00 sec) mysql> select * from students; +------+------+------+-------+ | id | name | age | score | +------+------+------+-------+ | 1001 | TOM | 22 | 90.5 | +------+------+------+-------+ 1 row in set (0.00 sec) mysql>
-
- 多条数据插入
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insert into 表名(字段1, 字段2, 字段3, ...) values(值11, 值21, 值31, ...),(值12, 值22, 值32, ...),... - eg
-
mysql> insert into students(id,name,age,score) values(1002,'JACK',18,80.0),(1003,'LUCY',20,98.5); Query OK, 2 rows affected (0.00 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql> select * from students; +------+------+------+-------+ | id | name | age | score | +------+------+------+-------+ | 1001 | TOM | 22 | 90.5 | | 1002 | JACK | 18 | 80 | | 1003 | LUCY | 20 | 98.5 | +------+------+------+-------+ 3 rows in set (0.00 sec) mysql>
-
6.2 更新数据
- 更新特定数据
-
updata 表名 set 字段1=值1, 字段2=值2, ..., where 条件 - eg
-
mysql> select * from students; +------+--------+------+-------+ | id | name | age | score | +------+--------+------+-------+ | 1001 | TOM | 22 | 90.5 | | 1002 | JACK | 18 | 80 | | 1003 | LUCY | 16 | 100 | | 1004 | 曹操 | 18 | 80 | +------+--------+------+-------+ 4 rows in set (0.00 sec) mysql> update students set age=20,score=99.0 where id=1003; Query OK, 1 row affected (0.01 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from students; +------+--------+------+-------+ | id | name | age | score | +------+--------+------+-------+ | 1001 | TOM | 22 | 90.5 | | 1002 | JACK | 18 | 80 | | 1003 | LUCY | 20 | 99 | | 1004 | 曹操 | 18 | 80 | +------+--------+------+-------+ 4 rows in set (0.00 sec) mysql>
-
- 更新所有数据
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update 表名 set 字段1=值1, 字段2=值2, ... - eg
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mysql> select * from students; +------+--------+------+-------+ | id | name | age | score | +------+--------+------+-------+ | 1001 | TOM | 22 | 90.5 | | 1002 | JACK | 18 | 80 | | 1003 | LUCY | 20 | 99 | | 1004 | 曹操 | 18 | 80 | +------+--------+------+-------+ 4 rows in set (0.00 sec) mysql> update students set age=20,score=80; Query OK, 4 rows affected (0.00 sec) Rows matched: 4 Changed: 4 Warnings: 0 mysql> select * from students; +------+--------+------+-------+ | id | name | age | score | +------+--------+------+-------+ | 1001 | TOM | 20 | 80 | | 1002 | JACK | 20 | 80 | | 1003 | LUCY | 20 | 80 | | 1004 | 曹操 | 20 | 80 | +------+--------+------+-------+ 4 rows in set (0.00 sec) mysql>
-
6.3 删除数据
- 删除指定数据
-
delete from 表名 where 条件 - eg
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mysql> select * from students; +------+--------+------+-------+ | id | name | age | score | +------+--------+------+-------+ | 1001 | TOM | 20 | 80 | | 1002 | JACK | 20 | 80 | | 1003 | LUCY | 20 | 80 | | 1004 | 曹操 | 20 | 80 | +------+--------+------+-------+ 4 rows in set (0.00 sec) mysql> delete from students where name='曹操'; Query OK, 1 row affected (0.00 sec) mysql> select * from students; +------+------+------+-------+ | id | name | age | score | +------+------+------+-------+ | 1001 | TOM | 20 | 80 | | 1002 | JACK | 20 | 80 | | 1003 | LUCY | 20 | 80 | +------+------+------+-------+ 3 rows in set (0.00 sec) mysql>
-
- 删除所有数据
-
delete from 表名
-
6.4 查询数据
- 我先贴一下原始数据，下面的所有数据查询结果都可以跟原始数据比较下
-
+------+------+------+------+-------+-------+ | id | name | sex | age | score | class | +------+------+------+------+-------+-------+ | 1001 | TOM | boy | 18 | 80 | 1 | | 1002 | JACK | boy | 20 | 85.5 | 1 | | 1003 | LUCY | girl | 20 | 10.5 | 2 | | 1004 | ANDY | girl | 20 | 82.2 | 1 | | 1005 | BOB | boy | 19 | 86.9 | 2 | | 1006 | AMY | girl | 18 | 70.5 | 1 | | 1007 | EMMY | girl | 19 | 99.5 | 2 | | 1008 | DAWN | boy | 21 | 95.5 | 2 | +------+------+------+------+-------+-------+
6.4.1 简单数据查询
- 查询所有字段
-
select * from 表名 - eg
-
mysql> select * from students; +------+------+------+------+-------+-------+ | id | name | sex | age | score | class | +------+------+------+------+-------+-------+ | 1001 | TOM | boy | 18 | 80 | 1 | | 1002 | JACK | boy | 20 | 85.5 | 1 | | 1003 | LUCY | girl | 20 | 10.5 | 2 | | 1004 | ANDY | girl | 20 | 82.2 | 1 | | 1005 | BOB | boy | 19 | 86.9 | 2 | | 1006 | AMY | girl | 18 | 70.5 | 1 | | 1007 | EMMY | girl | 19 | 99.5 | 2 | | 1008 | DAWN | boy | 21 | 95.5 | 2 | +------+------+------+------+-------+-------+ 8 rows in set (0.00 sec) mysql>
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- 查询指定字段
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select 字段1,字段2,... from 表名 - 比如我们只查询id，姓名和班级
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mysql> select id,name,class from students; +------+------+-------+ | id | name | class | +------+------+-------+ | 1001 | TOM | 1 | | 1002 | JACK | 1 | | 1003 | LUCY | 2 | | 1004 | ANDY | 1 | | 1005 | BOB | 2 | | 1006 | AMY | 1 | | 1007 | EMMY | 2 | | 1008 | DAWN | 2 | +------+------+-------+ 8 rows in set (0.00 sec) mysql>
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- 避免重复数据查询
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select distinct 字段1,字段2,... from 表名 - 比如数据表中学生数据可能有上千条，但班级可能就几十个，我们想查询都有哪些班级。就可以去掉重复信息查询。
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mysql> select distinct class from students; +-------+ | class | +-------+ | 1 | | 2 | +-------+ 2 rows in set (0.00 sec) mysql>
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- 实现数字四则运算数据查询
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select 字段1 运算符 运算符操作数, 字段2 运算符 运算符操作数, ... from 表名 - 运算符包括：+ - * / %
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mysql> select id + 1, name from students; +--------+------+ | id + 1 | name | +--------+------+ | 1002 | TOM | | 1003 | JACK | | 1004 | LUCY | | 1005 | ANDY | | 1006 | BOB | | 1007 | AMY | | 1008 | EMMY | | 1009 | DAWN | +--------+------+ 8 rows in set (0.00 sec) mysql>
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- 给字段起别名
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select 字段1 as 字符串, 字段2 as 字符串, ... from 表名 - eg
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mysql> select name as '姓名', sex as '性别', age as '年龄', score as '分数', class as '班级' from students; +--------+--------+--------+--------+--------+ | 姓名 | 性别 | 年龄 | 分数 | 班级 | +--------+--------+--------+--------+--------+ | TOM | boy | 18 | 80 | 1 | | JACK | boy | 20 | 85.5 | 1 | | LUCY | girl | 20 | 10.5 | 2 | | ANDY | girl | 20 | 82.2 | 1 | | BOB | boy | 19 | 86.9 | 2 | | AMY | girl | 18 | 70.5 | 1 | | EMMY | girl | 19 | 99.5 | 2 | | DAWN | boy | 21 | 95.5 | 2 | +--------+--------+--------+--------+--------+ 8 rows in set (0.00 sec) mysql>
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6.4.2 条件数据查询
-
-
select 字段 from 表名 where 条件 - 条件可使用关系运算符和逻辑运算符
- 关系运算符：> >= < <= = !=
- 逻辑运算符：and or not
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- 单条件数据查询
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select 字段 from 表名 where 条件 - 比如我们只查询1班的学生信息
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mysql> select * from students where class=1; +------+------+------+------+-------+-------+ | id | name | sex | age | score | class | +------+------+------+------+-------+-------+ | 1001 | TOM | boy | 18 | 80 | 1 | | 1002 | JACK | boy | 20 | 85.5 | 1 | | 1004 | ANDY | girl | 20 | 82.2 | 1 | | 1006 | AMY | girl | 18 | 70.5 | 1 | +------+------+------+------+-------+-------+ 4 rows in set (0.00 sec) mysql>
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- 多条件数据查询
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select 字段 from 表名 where 条件1 and 条件2 - 比如我们查询1班女生的学生信息
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mysql> select * from students where class=1 and sex='girl'; +------+------+------+------+-------+-------+ | id | name | sex | age | score | class | +------+------+------+------+-------+-------+ | 1004 | ANDY | girl | 20 | 82.2 | 1 | | 1006 | AMY | girl | 18 | 70.5 | 1 | +------+------+------+------+-------+-------+ 2 rows in set (0.00 sec) mysql>
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- 符合范围的数据查询
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select 字段 from 表名 where 字段名 between 值1 and 值2 - 比如我们要查询分数在80到90区间段的学生信息
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mysql> select * from students where score between 80 and 90; +------+------+------+------+-------+-------+ | id | name | sex | age | score | class | +------+------+------+------+-------+-------+ | 1001 | TOM | boy | 18 | 80 | 1 | | 1002 | JACK | boy | 20 | 85.5 | 1 | | 1004 | ANDY | girl | 20 | 82.2 | 1 | | 1005 | BOB | boy | 19 | 86.9 | 2 | +------+------+------+------+-------+-------+ 4 rows in set (0.00 sec) mysql>
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- 不符合范围的数据查询
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select 字段 from 表名 where 字段名 not between 值1 and 值2 - 我们要查询分数不在80到90区间段的学生信息
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mysql> select * from students where score not between 80 and 90; +------+------+------+------+-------+-------+ | id | name | sex | age | score | class | +------+------+------+------+-------+-------+ | 1003 | LUCY | girl | 20 | 10.5 | 2 | | 1006 | AMY | girl | 18 | 70.5 | 1 | | 1007 | EMMY | girl | 19 | 99.5 | 2 | | 1008 | DAWN | boy | 21 | 95.5 | 2 | +------+------+------+------+-------+-------+ 4 rows in set (0.00 sec) mysql>
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- 空值查询
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select 字段 from 表名 where 字段名 is null - 我们查询下字段为空的数据。因为数据里没有空值，所以查询结果为空。
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mysql> select * from students where id is null; Empty set (0.00 sec)
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- 非空值查询
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select 字段 from 表名 where 字段名 is not null -
mysql> select * from students where id is not null; +------+------+------+------+-------+-------+ | id | name | sex | age | score | class | +------+------+------+------+-------+-------+ | 1001 | TOM | boy | 18 | 80 | 1 | | 1002 | JACK | boy | 20 | 85.5 | 1 | | 1003 | LUCY | girl | 20 | 10.5 | 2 | | 1004 | ANDY | girl | 20 | 82.2 | 1 | | 1005 | BOB | boy | 19 | 86.9 | 2 | | 1006 | AMY | girl | 18 | 70.5 | 1 | | 1007 | EMMY | girl | 19 | 99.5 | 2 | | 1008 | DAWN | boy | 21 | 95.5 | 2 | +------+------+------+------+-------+-------+ 8 rows in set (0.00 sec) mysql>
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- 带in关键字的集合查询
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select 字段 from 表名 where 字段名 in(值1, 值2, ...) - 我们只查询TOM和JACK的信息
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mysql> select * from students where name in('TOM','JACK'); +------+------+------+------+-------+-------+ | id | name | sex | age | score | class | +------+------+------+------+-------+-------+ | 1001 | TOM | boy | 18 | 80 | 1 | | 1002 | JACK | boy | 20 | 85.5 | 1 | +------+------+------+------+-------+-------+ 2 rows in set (0.00 sec) mysql>
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- 不在集合中的数据查询
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select 字段 from 表名 where 字段名 not in(值1, 值2, ...) - 查询除了TOM和JACK的其他学生信息
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mysql> select * from students where name not in('TOM','JACK'); +------+------+------+------+-------+-------+ | id | name | sex | age | score | class | +------+------+------+------+-------+-------+ | 1003 | LUCY | girl | 20 | 10.5 | 2 | | 1004 | ANDY | girl | 20 | 82.2 | 1 | | 1005 | BOB | boy | 19 | 86.9 | 2 | | 1006 | AMY | girl | 18 | 70.5 | 1 | | 1007 | EMMY | girl | 19 | 99.5 | 2 | | 1008 | DAWN | boy | 21 | 95.5 | 2 | +------+------+------+------+-------+-------+ 6 rows in set (0.00 sec) mysql>
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- 带like关键字的模糊查询
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select 字段 from 表名 where 字段名 like 值 - “%” 通配任意多字符，"_"通配一个字符
- 我们查询下姓名以字母A开头的学生信息和姓名以字母A开头且只有3个字母组成的学生信息
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mysql> select * from students where name like 'A%'; +------+------+------+------+-------+-------+ | id | name | sex | age | score | class | +------+------+------+------+-------+-------+ | 1004 | ANDY | girl | 20 | 82.2 | 1 | | 1006 | AMY | girl | 18 | 70.5 | 1 | +------+------+------+------+-------+-------+ 2 rows in set (0.00 sec) mysql> select * from students where name like 'A__'; +------+------+------+------+-------+-------+ | id | name | sex | age | score | class | +------+------+------+------+-------+-------+ | 1006 | AMY | girl | 18 | 70.5 | 1 | +------+------+------+------+-------+-------+ 1 row in set (0.00 sec) mysql>
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6.4.3 排序查询
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select 字段名 from 表名 order by 字段名 asc/desc - asc : 升序， desc : 降序
- 以成绩降序查询学生信息
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mysql> select * from students order by score desc; +------+------+------+------+-------+-------+ | id | name | sex | age | score | class | +------+------+------+------+-------+-------+ | 1007 | EMMY | girl | 19 | 99.5 | 2 | | 1008 | DAWN | boy | 21 | 95.5 | 2 | | 1005 | BOB | boy | 19 | 86.9 | 2 | | 1002 | JACK | boy | 20 | 85.5 | 1 | | 1004 | ANDY | girl | 20 | 82.2 | 1 | | 1001 | TOM | boy | 18 | 80 | 1 | | 1006 | AMY | girl | 18 | 70.5 | 1 | | 1003 | LUCY | girl | 20 | 10.5 | 2 | +------+------+------+------+-------+-------+ 8 rows in set (0.00 sec) mysql>
6.4.4 分组查询
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select 字段名 from 表名 group by 字段名 - 分组后再进行条件过滤
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select 字段名 from 表名 group by 字段名 having 条件 -
mysql> select class from students group by class; +-------+ | class | +-------+ | 1 | | 2 | +-------+ 2 rows in set (0.00 sec) mysql> - 是不是感觉分组查询没啥用？别着急，分组是为了统计，所以一般是和统计函数一起使用的
- 统计函数
- count:统计表中记录的条数
- avg:统计计算字段的平均值
- sum:统计字段的总和
- max:查询字段的最大值
- min:查询字段的最小值
- 比如我们统计1班和2班分别有多少人
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mysql> select class,count(class) from students group by class; +-------+--------------+ | class | count(class) | +-------+--------------+ | 1 | 4 | | 2 | 4 | +-------+--------------+ 2 rows in set (0.00 sec) mysql> - 统计1班和2班的平均分
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mysql> select class,avg(score) from students group by class; +-------+------------+ | class | avg(score) | +-------+------------+ | 1 | 79.55 | | 2 | 73.1 | +-------+------------+ 2 rows in set (0.00 sec) mysql> - 只统计1班平均分
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mysql> select class,avg(score) from students group by class having class=1; +-------+------------+ | class | avg(score) | +-------+------------+ | 1 | 79.55 | +-------+------------+ 1 row in set (0.00 sec) mysql>