适合新手——MySQL中基于SQL语言增删改查等基础的练习大汇总

前言

这是我听老师讲课做的笔记,考试要看的。 这是视频地址
作者：RodmaChen
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配套这篇文章解释更香哇！MySQL中SQL数据库基础知识大汇总出炉了

一. DDL——数据库、表、视图等的建立、删除

接下来如何创建表结构

1. 设置id主键：id int not null auto_increment primary key（auto_increment顺序增加）
2. 设置名字：name varchar(30)
3. 设置电话：phone varchar(20)

create table contacts( 
ip int not null auto_increment primary key , 
name varchar(30) ,  
phone varchar(20) );

添加字段sex，类型为varchar(1)

alter table contacts add sex varchar(1);

修改字段sex的类型为tinyint

alter table contacts modify sex tinyint;

删除字段sex

alter table contacts drop column sex;

删除contacts表

drop table contacts;

二. DML——添加、删除和修改数据表中的记录

测试：

1. 查看：select * from contacts;

2. 插入：

insert into contacts (name, phone) values ("my'love",'108944120');


insert into contacts(name,phone ) value ('啊陈','108944121');

更新

 update contacts set phone='110998'
 update contacts set phone='110998' where name='啊陈';

删除

delete from contacts where name='啊陈';

三. DCL——数据库对象的权限管理和事务管理

3.1 用户管理——user

\G格式化输入

用户名和密码对了主机不对也不能连

create user 'chen'@'%' identified by '123456';

create user 'yun'@'192.168.0.1' identified by '123456';

再打开另一个终端登录用户，观察是否能登录


删除

mysql> drop user 'chen'@'%';

mysql> drop user 'yun'@'192.168.0.1';

修改密码

alter user '用户名'@'主机名' identified by '新密码';

3.2 权限管理

给用户赋予了mydb所有表权限

//只有登录的权限
mysql> flush privileges;

//给用户赋予了mydb所有表权限
mysql> grant select on mydb.* to 'chen'@'%';

//在查看发现有了select只读权限了

//删除
mysql> revoke select on mydb.* from 'chen'@'%';
//刷新权限
mysql> flush privileges;

3.3 禁止root用户远程登录

mysql> use mysql;

mysql> select user,host from user;

如果出现user不是root，host不是localhost的情况root用户就能远程登录
%如何机器都能远程登录

改为只能 本机连

update user set host='localhost' where user ='chen';

mysql> select user,host from user;

3.4 忘记用户密码的解决

停掉服务

不能关闭窗口

//停止服务
net stop mysql80
//关闭权限验证
mysqld --defaults-file="C:\ProgramData\MySQL\MySQL Server 8.0\my.ini" --console --skip-grant-tables --shared-memory

重新开一个窗口

//使用数据表
mysql> use mysql;
//查看数据表
mysql> show tables;
//user里面包含用户，查看用户
mysql> select user,host from user;

////刷新权限
flush privileges;
//更改root用户密码
alter user 'root'@'localhost' identified by '123456';

//重新启动服务
C:\WINDOWS\system32>net start mysql80

//登录进去就行
C:\WINDOWS\system32>mysql -u root -p

四. DQL——查询数据表中的记录

4.1 select简单查询

mysql> select * from contacts;
mysql> select name ,phone from contacts;
mysql> select name , phone from contacts  where name="my'love";

也可以当成计算器使用

4.2 where条件查询

create table employee( 	id int not null auto_increment primary key, 	
name varchar(30) comment '姓名', 	
sex varchar(1) comment '性别', 	
salary int comment '薪资（元）' );

insert into employee(name, sex, salary) values('张三', '男', 5500); 
insert into employee(name, sex, salary) values('李洁', '女', 4500); 
insert into employee(name, sex, salary) values('李小梅', '女', 4200); 
insert into employee(name, sex, salary) values('欧阳辉', '男', 7500); 
insert into employee(name, sex, salary) values('李芳', '女', 8500); 
insert into employee(name, sex, salary) values('张江', '男', 6800); 
insert into employee(name, sex, salary) values('李四', '男', 12000); 
insert into employee(name, sex, salary) values('王五', '男', 3500); 
insert into employee(name, sex, salary) values('马小龙', '男', 6000); 
insert into employee(name, sex, salary) values('龙五', '男', 8000); 
insert into employee(name, sex, salary) values('冯小芳', '女', 10000); 
insert into employee(name, sex, salary) values('马小花', '女', 4000);

select * from employee where sex = '男'  and  salary >= 10000;

select * from employee where sex = '男' or salary >= 10000;

select * from employee where sex = '男' and  salary <= 4000 or salary >=10000;

4.2.1 in的用法

select * from employee where id=1 or id =2 or id =3;
select * from employee where id in(1,2,3,4);

4.2.2 like用法

select * from employee where name like '张三';

select * from employee where name like '李%';

4.3 函数

mysql> select now（）;

4.3.1 聚合函数

mysql> select count(*) from employee;
mysql> select count(*) from employee where sex ='男';
mysql> select count(*) from employee where sex ='女';
mysql> select sum(salary) from employee;
mysql> select avg(salary) from employee;
mysql> select max(salary) from employee;
mysql> select min(salary) from employee;

为什么会忽略空值，十三个员工平均值不变，会跳过这个员工

4.3.2 case when判断条件语句

select
id,
name,
case sex
	when '男' then 'A'
	when '女' then 'M'
	else ''
end as sex,
salary
from employee;

4.4 查询结果排序与分页

按某数值排序，从高到低，从大到小等等

4.4.1 order by 排序

实例：
薪资排序：salary，降序后面加desc

mysql> select * from employee order by salary;

按照sex性别排序后薪资salary排序

select * from employee order by sex ,salary desc;

4.4.2 limit 分页查询

获取记录：

mysql> select * from employee limit 2;

分页：

mysql> select * from employee limit 3,3 ;

4.5 数据的分组和筛选
4.5.1 group by——分组

4.5 数据的分组和筛选

4.5.1 group by——分组

select sex ,count(*) from employee group by sex;

select sex ,sum(salary) from employee group by sex;

4.5.2 having——筛选各组数据

having小于等于五的用法

mysql> select sex , count(*)  from employee group by  sex having count(*)<=5;

4.5.3 group_concat——显示统计数量列表

mysql> select sex ,count(*) ,group_concat(name) from
employee group by sex;

select sex,count(*) , group_concat(name order by name desc) from employee group by sex;


select sex, count(*),group_concat(name order by name desc separator ';') from employee group by sex;

4.6 去除重复数据——distinct

数据表

create table footprint(
	id int not null auto_increment primary key,
	username varchar(30) comment '用户名',
	city varchar(30) comment '城市',
	visit_date varchar(10) comment '到访日期'
);

insert into footprint(username, city, visit_date) values('liufeng', '贵阳', '2019-12-05');
insert into footprint(username, city, visit_date) values('liufeng', '贵阳', '2020-01-15');
insert into footprint(username, city, visit_date) values('liufeng', '北京', '2018-10-10');
insert into footprint(username, city, visit_date) values('zhangsan', '上海', '2020-01-01');
insert into footprint(username, city, visit_date) values('zhangsan', '上海', '2020-02-02');
insert into footprint(username, city, visit_date) values('lisi', '拉萨', '2016-12-20');

//看用户distinct
mysql> select distinct username from footprint;
//group by也可以去重，不过它做的是分组统计的，distinct只做去重
mysql> select username from footprint group by username;
//每一个用户去过的城市
mysql> select distinct username ,city from footprint;

4.7 表连接（内连接、外连接、自连接）

多张表，表连接
主要学习内连接和左连接

drop table if exists score;//数据表
drop table if exists student;//学生表

create table student(
	stu_no varchar(20) not null primary key comment '学号',
	name varchar(30) comment '姓名',
	address varchar(150) comment '地址'
);

insert into student(stu_no, name, address) values('2016001', '张三', '贵州贵阳');
insert into student(stu_no, name, address) values('2016002', '李芳', '陕西兴平');
insert into student(stu_no, name, address) values('2016003', '张晓燕', '江西南昌');

create table score(
	id int not null auto_increment primary key,
	course varchar(50) comment '科目',
	stu_no varchar(20) comment '学号',
	score int comment '分数',
	foreign key(stu_no) references student(stu_no)
);

insert into score(course, stu_no, score) values('计算机', '2016001', 99);
insert into score(course, stu_no, score) values('离散数学', '2016001', 85);
insert into score(course, stu_no, score) values('计算机', '2016002', 78);

4.7.1 内连接

//内连接 join
select A.stu_no, A.name ,B.course,B.score
from student A join score B  on(A.stu_no=B.stu_no);

// inner join
select A.stu_no, A.name ,B.course,B.score
from student A inner  join score B  on(A.stu_no=B.stu_no);

// where
select A.stu_no, A.name ,B.course,B.score
from student A ,score B where  A.stu_no=B.stu_no;

4.7.2 左连接

//左连接
select A.stu_no, A.name ,B.course,B.score
from student A
left  join score B  on(A.stu_no=B.stu_no);

// 交叉连接：没有按照学号进行关联（没啥用）
select A.stu_no, A.name ,B.course,B.score
from student A ,score B;

4.7.3 自连接

自连接：有层次结构的表

drop table if exists area;
drop table if exists area;

create table area(
	id int not null auto_increment primary key comment '区域id',
	pid int not null comment '父id(0-省份)',
	name varchar(30) comment '区域名称'
);

insert into area(id, pid, name) values(1, 0, '贵州省');
insert into area(id, pid, name) values(2, 1, '贵阳');
insert into area(id, pid, name) values(3, 1, '遵义');
insert into area(id, pid, name) values(4, 0, '广东省');
insert into area(id, pid, name) values(5, 4, '广州');
insert into area(id, pid, name) values(6, 4, '深圳');

// 父类不等于0的子类
mysql> select * from area where pid<>0;

// 父类和子类的连接
mysql> select A.id , A.name, B.name as provinceName
    -> from area A,area B
    -> where A.pid=B.id and A.pid<>0;

4.8 子查询EXISTS和IN的使用

子查询 in ，exists场景不一样好处不一样

以下用的数据表是上面创建的学生表和数据表

查询所有选修课程的学生
select A.*
from student A
where A.stu_no in(select B.stu_no from score B);

查询选修了离散数学的学生
select A.*
from student A
where A.stu_no in(select B.stu_no from score B where B.course='离散数学');

查询所有选修了课程的学生
select A.*
from student A
where exists (select * from score B where A.stu_no =B.stu_no);

查询所有未选修课程的学生
select A.*
from student A
where not exists (select * from score B where A.stu_no=B.stu_no);

小知识

1. 查看数据目录

列：

mysql> show variables like '%datadir%';

//select abs(-10);绝对值
select abs(-10);
//求字符串长度
select length("cheng");
select length("cheng") from dual;
select * from dual;

3. 加密

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