MySQL 初探：从基础到优化

什么是 MySQL？

MySQL 是一个开源的关系型数据库管理系统 (RDBMS)，使用结构化查询语言 (SQL) 进行数据管理。作为最流行的数据库之一，MySQL 被广泛应用于各类网站和应用中，从小型应用到大型复杂系统。

MySQL 的特点

开源免费：尽管 MySQL 有商业版本，但其开源版本已经足够满足大部分应用需求。
跨平台：支持多种操作系统，包括 Windows、Linux 和 macOS。
高性能：通过优化的存储引擎如 InnoDB 提供高效的读写能力。
支持事务：事务管理使得 MySQL 能够确保数据一致性和完整性。
社区支持：拥有庞大的用户和开发者社区，提供丰富的资源和支持。

mac 下安装启动不了

sudo chown -R mysql /usr/local/mysql/data
sudo /usr/local/mysql/support-files/mysql.server start

navcat 连接不上 mysql

https://blog.csdn.net/dabao87/article/details/80571877

mysql 基本操作

显示所有数据库

show databases;

进入某人数据库

use databases;

查看数据库里的表

show tables;

查看在某个数据库

slect database()

创建一个表

create table info(id int name varchar(20));

查看表结构

desc ifof(表名)

查看数据

select * from info(表名)

查看数据库版本

select version()

mysql 语法规范

不区分大小写，但关建议关键字，表名，列名小写
每条命令最好用分号结尾
每条命令根据需要，可以进行缩进或换行

DQL 查询语言

查询表中单个字段

SELECT last_name (字段名) FROM employees;

查询表中多个字段

SELECT last_name,salary,email from employees;

查询表中所有字段

SELECT * FROM employees;

查询常量值

SELECT 100;

查询表过式

SELECT 100 * 98;

查询函数

SELECT VERSION()

起别名

SELECT 100%92 as 结果;
SELECT last_name as 姓,first_name as 名 FROM employees;

去重

SELECT DISTINCT department_id from employees

字符串的拼接

SELECT CONCAT(last_name,first_name) as 姓名 FROM employees;

条件查询

按条件表过式筛选

SELECT * FROM employees WHERE salary > 12000

按逻辑运算符进行筛选

SELECT last_name,salary,commission_pct FROM employees WHERE salary >=10000 AND salary <=20000;
SELECT * FROM employees WHERE NOT(department_id >=90 AND department_id <=110) OR salary >15000;

模糊查询 like between and in is null is not null

SELECT * FROM employees WHERE last_name LIKE '%a%';

查询两者之间 between and 包含临界值，临界值的顺序不能错序

SELECT * FROM employees WHERE employee_id BETWEEN 100 AND 120;

in 判断某个字段的值是否属于 in 列表中的某一项

SELECT last_name,job_id FROM employees WHERE job_id IN('IT_PROT','AD_VP','AD_PRES');

is null = 或<>不能用于判断 null 值

SELECT last_name,commission_pct FROM employees WHERE commission_pct IS NULL

排序查询

从高到低进行排序

SELECT * FROM employees ORDER BY salary DESC;

排序+筛选

SELECT * FROM employees WHERE department_id >=90 ORDER BY hiredate ASC

按字节进行排序

SELECT LENGTH(last_name) last_name,salary FROM employees ORDER BY LENGTH(last_name) DESC;

模糊查询排序

SELECT *,LENGTH(email) FROM employees WHERE email LIKE '%e%' ORDER BY LENGTH(email) DESC, department_id ASC;

字符函数

获取字符串的字节个数

SELECT LENGTH("hello world");

拼接字符口串

SELECT CONCAT(last_name,'_',first_name) as 姓名 FROM employees;

upper,lower 将字符串变大写，小写

SELECT CONCAT(UPPER(last_name),UPPER(first_name)) as 姓名 FROM employees;

substr 截取从指定索引处后面的所有字符

SELECT SUBSTR("李莫悉爱上了陆展元",7) as out_put;
SELECT SUBSTR("李莫悉爱上了陆展元",1,3) as out_put;

instr 返回子串第一次出现的索引，如果找不到返回 0

SELECT INSTR('杨不悔爱上了六侠','六侠') as out_put;

trim

lpad 用指定的字符实现左填充指定长度

SELECT LPAD("刘德华",10,"*") as out_put;

rpad 用指定的字符实现右填充指定的长度

SELECT RPAD("刘德华",10,"*") as out_put;

replace 替换

SELECT REPLACE("张无忌爱上周芷若","周芷若","赵敏") as out_put;

常用数学函当我

round 四舍五入

SELECT ROUND(1.55) as out_put;

ceil 向上取整

SELECT CEIL(1.01) as out_put;

floor 向下取整

SELECT FLOOR(1.65) as out_out;

truncate 截断

SELECT TRUNCATE(1.65,1) as out_put;

mod 取余

SELECT MOD(10,3) as out_put;

日期函数

now 返回当前系统日期+时间

SELECT NOW() as out_put;

curdate 返回当前系统日期，不包含时间

SELECT CURDATE();

curtime 返回当前日间，不包含日期

SELECT curtime();

指定返回年，月，日，时，分，秒

SELECT YEAR(NOW()) as out_put;

流程控制函数

if 函数

SELECT IF(10 > 5,'大','小');
SELECT last_name,commission_pct,IF(commission_pct IS NULL,'没奖金,呵呵','有奖金,嘻嘻')  备注 FROM employees;

case 函数

SELECT salary 原始工资,department_id,
CASE department_id
	WHEN 20 THEN salary * 1.1
	WHEN 40 THEN salary * 1.2
	WHEN 50 THEN salary * 1.3
	ELSE salary
END AS 新工资 FROM employees;

分组函数

sum 求合

SELECT SUM(salary) FROM employees;

avg 求平均值

SELECT AVG(salary) FROM employees;

distinct 去重

SELECT SUM(DISTINCT salary) FROM employees;

count 统计

SELECT COUNT(DISTINCT salary) FROM employees;

求最大值，最小值，总合

SELECT MAX(salary) as 最大值,MIN(salary) as 最小值,SUM(salary) as 总合 from employees;

求两个时间相差的天数

SELECT DATEDIFF(MAX(hiredate),MIN(hiredate)) as 差异 FROM employees;

统计某个部门 id 的个数

SELECT COUNT(*) 个数 FROM employees WHERE department_id = 90;

group by 函数

查询每个工种的最高工资

SELECT MAX(salary),job_id FROM employees GROUP BY job_id;

查询每个位置上的部门数

SELECT COUNT(*),location_id FROM departments GROUP BY location_id;

查询每个部门的平均工资

SELECT AVG(salary),department_id FROM employees GROUP BY department_id;

查询有奖金的每个领导手下员工的最高工资

SELECT MAX(salary),manager_id FROM employees WHERE commission_pct IS NOT NULL GROUP BY manager_id;

查询每个部门员工数大于 2 的

SELECT COUNT(*),department_id FROM employees GROUP BY department_id HAVING COUNT(*) > 2

join 连接查询

查询员工名对应的部门名

SELECT last_name,department_name FROM employees,departments WHERE employees.department_id = departments.department_id;

查询员工名，工种号，工种名

SELECT last_name,employees.job_id,job_title FROM employees,jobs WHERE employees.job_id = jobs.job_id;

查询有奖金的员工名，部门名

SELECT last_name,department_name,commission_pct FROM employees e,departments d WHERE e.department_id = d.department_id AND e.commission_pct IS NOT NULL;

查询城市名第二个字符为 o 的部门名和城市名

SELECT department_name,city FROM departments d,locations l WHERE d.location_id = l.location_id AND city LIKE '_o%';

查询每个城市的部门个数

SELECT COUNT(*) AS 个数,city FROM departments d,locations l WHERE d.location_id = l.location_id GROUP BY city;

查询每个工种的工种名和员工个数，并且按员工个数降序

SELECT job_title,COUNT(*) FROM employees e,jobs j WHERE e.job_id = j.job_id GROUP BY job_title ORDER BY COUNT(*) DESC;

等值链查询员工名，部门名

SELECT last_name,department_name FROM employees e INNER JOIN departments d ON e.department_id = d.department_id;

查询名字中包含 e 的员工名和工种名

SELECT last_name,job_title FROM employees e INNER JOIN jobs j ON e.job_id = j.job_id WHERE e.last_name LIKE '%e%';

查询部门数大于 3 的城市名和部门数

SELECT city,COUNT(*) AS 部门个数 FROM departments AS d INNER JOIN locations AS l ON d.location_id = l.location_id GROUP BY city HAVING COUNT(*) > 3;

日期与时间

SELECT CURDATE(),CURRENT_DATE(), CURTIME(), NOW(),SYSDATE(),UTC_DATE(),UTC_TIME() FROM DUAL;

日期与时间 cho

SELECT UNIX_TIMESTAMP();
SELECT FROM_UNIXTIME(1702247160)

子查询

标是子查询查询工资比 Abel 高的

SELECT * FROM employees WHERE salary > (SELECT salary FROM employees WHERE last_name = "Abel");

返回公司工资最低的员工的 last_name,job_id 和 salary

SELECT last_name,job_id,salary FROM employees
WHERE salary = (SELECT MIN(salary) FROM employees);

SELECT MIN(salary),department_id FROM employees
GROUP BY department_id
HAVING MIN(salary) > (SELECT MIN(salary) FROM employees WHERE department_id = 50);

行子查询

分页查询

查询前 5 条员工信息

SELECT * FROM employees LIMIT 0, 5;

查询第 11 条到第 25 条

SELECT * FROM employees LIMIT 10, 15;

DDL 语句

插入语法

插入的值的类型要与列的类型兼容或一致

INSERT INTO beauty(id,`name`,sex,borndate,phone,photo,boyfriend_id) VALUES(13,'刘德华','男','1789-4-23','15083356190',NULL,2)

插入方式 2

INSERT INTO beauty SET id = 15,`name` = "刘涛", borndate = "1978-09-10", phone = "15082256191"

修改语法

修改单表的记录

UPDATE beauty SET `name` = "阿涛" WHERE id = 15
UPDATE beauty SET phone = "15083356190" WHERE `name` LIKE "阿%"

删除语语

单表的删除

DELETE FROM beauty WHERE id = 15;

删除手机号为 9 结层的用户信息

DELETE FROM beauty WHERE phone LIKE '%9';

多表连接删除

DELETE b FROM beauty b INNER JOIN boys bo NO b.boyfriend_id = bo.id WHERE bo.boyName = "张无忌";

创建表

CREATE TABLE my_employess(
	id INT(10),
	first_name VARCHAR(10),
	last_name VARCHAR(10),
	salary DOUBLE(10,2)
);

DDL 数据定义语言

创建数据库

CREATE DATABASE books;
CREATE DATABASE IN NOT EXISTS books;

删除库

DROP DATABASE books;

创建表

CREATE TABLE book(
	id INT,
	bname VARCHAR(25),
	price DOUBLE,
	author VARCHAR(20)
);

修改表的字段名

ALTER TABLE book CHANGE COLUMN author pub_author  VARCHAR(20);

添加表的新列

ALTER TABLE book ADD COLUMN salary DOUBLE;

删除表的一列

ALTER TABLE book DROP COLUMN salary;

修改表名

ALTER TABLE book RENAME TO book_info;

表的删除

DROP TABLE book_info;

复制表的结构

CREATE TABLE copy LIKE book;

复制表的结构与数据

CREATE TABLE copy2 SELECT * FROM book;

sql 的执行过程

form ... -> on -> (left/ right join) -> where -> group by -> having ->
select -> distinct -> order by -> limit

外键约束

添加外键关联

ALTER TABLE emp add CONSTRAINT fk_emp_dept_id FOREIGN KEY(dept_id) REFERENCES dept(id);

删除外键

ALTER TABLE emp drop FOREIGN KEY fk_emp_dept_id

表结构多对多

CREATE TABLE users(
	id int PRIMARY KEY auto_increment COMMENT "主键",
	uname VARCHAR(10) NOT NULL UNIQUE COMMENT "姓名",
	age int CHECK(age > 0 && age < 120) COMMENT "年龄",
	ustatus CHAR(1) DEFAULT '1' COMMENT "状态",
	gender char(1) COMMENT "性别"
) COMMENT "用户表"


DROP TABLE users;


DESC users;

SELECT * FROM users;

INSERT INTO users(uname,age,ustatus,gender) VALUES("tom1",19,'1',"男"),("tom2",25,'0',"男")

SELECT * FROM users;

INSERT INTO users(uname,age,ustatus,gender) VALUES("tom3",119,'0','女');

SELECT * FROM users;

INSERT INTO users(uname,age,gender) VALUES("tom2",20,"女");

SELECT * FROM users;


CREATE TABLE dept(
	id int PRIMARY KEY auto_increment COMMENT "id",
	dname VARCHAR(50) NOT NULL COMMENT "部门名称"
) COMMENT "部门表"

INSERT INTO dept(id,dname) VALUES(1,"研发部"),(2,"市场部"),(3,"财务部"),(4,"销售部"),(5,"总经办");

CREATE TABLE emp(
	id INT PRIMARY KEY auto_increment COMMENT "id",
	ename VARCHAR(50) NOT NULL COMMENT "姓名",
	age int COMMENT "年龄",
	job VARCHAR(20) COMMENT "职位",
	salary INT COMMENT "薪资",
	entrydate date COMMENT "入职时间",
	managerid INT COMMENT "直属领导",
	dept_id INT COMMENT "部门is"
) COMMENT "员工表"

DROP TABLE emp;


INSERT INTO emp(ename,age,job,salary,entrydate,managerid,dept_id) VALUES
("金庸",66,"总载",20000,"2000-01-01",NULL,5),
("张小小",20,"项目经理",12500,"2005-12-05",1,1),
("杨逍",33,"开发",8400,"2000-11-03",2,1),
("韦一笑",48,"开发",11000,"2002-02-05",2,1),
("常遇春",43,"开发",10500,"2004-09-7",3,1),
("小昭",19,"程序员鼓励师",6000,"2004-10-12",2,1);

SELECT * FROM emp;

## 添加外键关联 
# ALTER TABLE emp add CONSTRAINT fk_emp_dept_id FOREIGN KEY(dept_id) REFERENCES dept(id);

ALTER TABLE emp ADD CONSTRAINT fk_emp_dept_id FOREIGN KEY(dept_id) REFERENCES dept(id);


## 删除外键
# ALTER TABLE emp drop FOREIGN KEY fk_emp_dept_id

ALTER TABLE emp drop FOREIGN KEY fk_emp_dept_id;

CREATE TABLE students(
	id int PRIMARY KEY auto_increment COMMENT "主键",
	sname VARCHAR(10) COMMENT "姓名",
	no VARCHAR(10) COMMENT "学号"
) COMMENT "学生表";

DROP TABLE students;

INSERT INTO students(sname,no)  VALUES
("黛绮丝","2000100101"),
("谢逊","2000100102"),
("天正","2000100103"),
("韦一笑","2000100104");


CREATE TABLE course(
	id INT PRIMARY KEY auto_increment COMMENT "主键id",
	cnme VARCHAR(10) COMMENT "课程名称"
) COMMENT "课程表";


INSERT INTO course(cnme) VALUES
("java"),
("php"),
("mysql"),
("hadoop");


CREATE TABLE students_course(
	id INT PRIMARY KEY auto_increment COMMENT "主键",
	studentid INT NOT NULL COMMENT "学生id",
	courseid INT NOT NULL COMMENT "课程id",
	CONSTRAINT fk_courseid FOREIGN KEY(courseid) REFERENCES course(id),
	CONSTRAINT fk_studentid FOREIGN KEY(studentid) REFERENCES students(id)
) COMMENT "学生课程中间表";

INSERT INTO students_course(studentid,courseid) VALUES
(1,1),
(1,2),
(1,3),
(2,3),
(3,4);

隐式内连接

SELECT * FROM emp,dept WHERE emp.dept_id = dept.id;

显示内连接

SELECT * FROM emp INNER JOIN dept ON emp.dept_id = dept.id;

外链接

SELECT * FROM emp;

左外链接

SELECT * FROM emp LEFT OUTER JOIN dept ON emp.dept_id = dept.id;

右外连接

SELECT * FROM emp RIGHT OUTER JOIN dept ON emp.dept_id = dept.id;

自连接

SELECT e.ename,b.ename FROM emp e,emp b WHERE e.managerid = b.id;

联合查询

SELECT * FROM emp WHERE salary < 8400 UNION SELECT * FROM emp WHERE age > 40;

标量子查询

SELECT * FROM emp WHERE dept_id = (SELECT id FROM dept WHERE dname = '研发部');

mysql 高级

Linux 安装 mysql

下载 mysql 的 repo 源

$ wget http://repo.mysql.com/mysql-community-release-el7-5.noarch.rpm

安装 mysql-community-release-el7-5.noarch.rpm 包

sudo rpm -ivh mysql-community-release-el7-5.noarch.rpm

安装这个包后，会获得两个 mysql 的 yum repo 源：/etc/yum.repos.d/mysql-community.repo，/etc/yum.repos.d/mysql-community-source.repo。

安装 mysql

sudo yum install mysql-server

根据步骤安装就可以了，不过安装完成后，没有密码，需要重置密码。

重置密码,重置密码前，首先要登录

mysql -u root

登录时有可能报这样的错：ERROR 2002 (HY000): Can‘t connect to local MySQL server through socket ‘/var/lib/mysql/mysql.sock‘ (2)，原因是/var/lib/mysql 的访问权限问题。下面的命令把/var/lib/mysql 的拥有者改为当前用户：

sudo chown -R openscanner:openscanner /var/lib/mysql

假如出现
chown: invalid user: ‘openscanner:openscanner’


chown root /var/lib/mysql/

重启服务

service mysqld restar

设置 mysql 密码

/usr/bin/mysqladmin -u root password tungee1024Zhang

mysql 安装位置

查看 mysql 安装位置

ps -ef | grep mysql

CREATE DATABASE iblog CHARACTER SET utf8 COLLATE utf8_general_ci;

mysql 高级

mysql 字符集

查看 mysql 字符集

show variables like '%character%';

设置为 utf8

vim /etc/my.cnf
character_set_server=utf8

重启 mysql

systemctl restart mysqld.service

用户管理

创建用户

CREATE USER 'zhangsan' IDENTIFIED by 'abc123';

修改密码

ALTER USER 'zhangdezhi' IDENTIFIED by 'tungee1024Zhang';

查看存储引擎

show engines;

查看默认存储引擎

select @@default_storage_engine;

mysql 高级

索引

创建索引

create index indexName on mytable(columname(length))

分析查询

explain select * from tbl_emp;

查询索引

show index from table

创建索引

create index idx_索引名称 on article(category_id,views);
ALTER  TABLE  book  ADD  INDEX  Y (`card`);

删除索引

drop index idx_索引名称 on table(表)

慢查询的开启

show variables like '%slow_query_log%';

MySQL 初探：从基础到优化

什么是 MySQL？

MySQL 的特点

mac 下安装启动不了

navcat 连接不上 mysql

mysql 基本操作

mysql 语法规范

DQL 查询语言

条件查询

排序查询

字符函数

常用数学函当我

日期函数

流程控制函数

分组函数

group by 函数

join 连接查询

日期与时间

子查询

分页查询

DDL 语句

插入语法

修改语法

删除语语

DDL 数据定义语言

sql 的执行过程

外键约束

添加外键关联

删除外键

表结构多对多

隐式内连接

显示内连接

外链接

左外链接

右外连接

自连接

联合查询

标量子查询

mysql 高级

Linux 安装 mysql

mysql 安装位置

mysql 高级

mysql 字符集

用户管理

查看存储引擎

查看默认存储引擎

mysql 高级

索引

创建索引

分析查询

查询索引

创建索引

删除索引

慢查询的开启

相关链接

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