适合新手——MySQL中基于SQL语言增删改查等基础的练习大汇总RodmaChen的博客-

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
关注我的csdn博客，更多Linux笔记知识还在更新
本人只在csdn写博客

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:ProgramDataMySQLMySQL Server 8.0my.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:WINDOWSsystem32>net start mysql80  //登录进去就行 C:WINDOWSsystem32>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

本人博客：https://blog.csdn.net/weixin_46654114
本人b站求关注：https://space.bilibili.com/391105864
转载说明：跟我说明，务必注明来源，附带本人博客连接。

请给我点个赞鼓励我吧