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第三章：数据库查询.md Normal file
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<h1><center>数据库查询</center></h1>
------
## 一：基本查询
#### 1.简介
​ 单表查询
​ 简单查询
​ 通过条件查询
​ 查询排序
​ 限制查询记录数
​ 使用集合函数查询
​ 分组查询
​ 使用正则表达式查询
#### 2.案例
创建案例所需表：company.employee5
```shell
雇员编号 id int
雇员姓名 name varchar(30)
雇员性别 sex enum
雇用时期 hire_date date
职位 post varchar(50)
职位描述 job_description varchar(100)
薪水 salary double(15,2)
办公室 office int
部门编号 dep_id int
```
```shell
MySQL [(none)]> CREATE TABLE company.employee5(
id int primary key AUTO_INCREMENT not null,
name varchar(30) not null,
sex enum('male','female') default 'male' not null,
hire_date date not null,
post varchar(50) not null,
job_description varchar(100),
salary double(15,2) not null,
office int,
dep_id int
);
```
插入模拟数据：
```shell
MySQL [(none)]> insert into company.employee5(name,sex,hire_date,post,job_description,salary,office,dep_id) values
('jack','male','20180202','instructor','teach',5000,501,100),
('tom','male','20180203','instructor','teach',5500,501,100),
('robin','male','20180202','instructor','teach',8000,501,100),
('alice','female','20180202','instructor','teach',7200,501,100),
('','male','20180202','hr','hrcc',600,502,101),
('harry','male','20180202','hr',NULL,6000,502,101),
('emma','female','20180206','sale','salecc',20000,503,102),
('christine','female','20180205','sale','salecc',2200,503,102),
('zhuzhu','male','20180205','sale',NULL,2200,503,102),
('gougou','male','20180205','sale','',2200,503,102);
```
语法格式：
​ select 字段名称,字段名称2...... from 表名 [条件]
##### a.简单查询
```shell
MySQL [company]> select * from employee5;
+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
| 1 | jack | male | 2018-02-02 | instructor | teach | 5000.00 | 501 | 100 |
| 2 | tom | male | 2018-02-03 | instructor | teach | 5500.00 | 501 | 100 |
| 3 | robin | male | 2018-02-02 | instructor | teach | 8000.00 | 501 | 100 |
| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
| 5 | | male | 2018-02-02 | hr | hrcc | 600.00 | 502 | 101 |
| 6 | harry | male | 2018-02-02 | hr | NULL | 6000.00 | 502 | 101 |
| 7 | emma | female | 2018-02-06 | sale | salecc | 20000.00 | 503 | 102 |
| 8 | christine | female | 2018-02-05 | sale | salecc | 2200.00 | 503 | 102 |
| 9 | zhuzhu | male | 2018-02-05 | sale | NULL | 2200.00 | 503 | 102 |
| 10 | gougou | male | 2018-02-05 | sale | | 2200.00 | 503 | 102 |
+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
10 rows in set (0.00 sec)
MySQL [company]> select name, salary, dep_id from employee5 where id <=5;
+-------+---------+--------+
| name | salary | dep_id |
+-------+---------+--------+
| jack | 5000.00 | 100 |
| tom | 5500.00 | 100 |
| robin | 8000.00 | 100 |
| alice | 7200.00 | 100 |
| | 600.00 | 101 |
+-------+---------+--------+
5 rows in set (0.00 sec)
```
##### b.避免重复
​ 不能部分使用DISTINCT，通常仅用于某一字段
```shell
MySQL [company]> SELECT post FROM employee5;
+------------+
| post |
+------------+
| instructor |
| instructor |
| instructor |
| instructor |
| hr |
| hr |
| sale |
| sale |
| sale |
| sale |
+------------+
10 rows in set (0.00 sec)
MySQL [company]> SELECT distinct post FROM employee5;
+------------+
| post |
+------------+
| instructor |
| hr |
| sale |
+------------+
3 rows in set (0.00 sec)
```
##### c.四则运算查询
```shell
MySQL [company]> SELECT name, salary, salary*14 FROM employee5;
+-----------+----------+-----------+
| name | salary | salary*14 |
+-----------+----------+-----------+
| jack | 5000.00 | 70000.00 |
| tom | 5500.00 | 77000.00 |
| robin | 8000.00 | 112000.00 |
| alice | 7200.00 | 100800.00 |
| | 600.00 | 8400.00 |
| harry | 6000.00 | 84000.00 |
| emma | 20000.00 | 280000.00 |
| christine | 2200.00 | 30800.00 |
| zhuzhu | 2200.00 | 30800.00 |
| gougou | 2200.00 | 30800.00 |
+-----------+----------+-----------+
10 rows in set (0.01 sec)
MySQL [company]> SELECT name, salary, salary*14 AS Annual_salary FROM employee5;
+-----------+----------+---------------+
| name | salary | Annual_salary |
+-----------+----------+---------------+
| jack | 5000.00 | 70000.00 |
| tom | 5500.00 | 77000.00 |
| robin | 8000.00 | 112000.00 |
| alice | 7200.00 | 100800.00 |
| | 600.00 | 8400.00 |
| harry | 6000.00 | 84000.00 |
| emma | 20000.00 | 280000.00 |
| christine | 2200.00 | 30800.00 |
| zhuzhu | 2200.00 | 30800.00 |
| gougou | 2200.00 | 30800.00 |
+-----------+----------+---------------+
10 rows in set (0.00 sec)
MySQL [company]> SELECT name, salary, salary*14 Annual_salary FROM employee5;
+-----------+----------+---------------+
| name | salary | Annual_salary |
+-----------+----------+---------------+
| jack | 5000.00 | 70000.00 |
| tom | 5500.00 | 77000.00 |
| robin | 8000.00 | 112000.00 |
| alice | 7200.00 | 100800.00 |
| | 600.00 | 8400.00 |
| harry | 6000.00 | 84000.00 |
| emma | 20000.00 | 280000.00 |
| christine | 2200.00 | 30800.00 |
| zhuzhu | 2200.00 | 30800.00 |
| gougou | 2200.00 | 30800.00 |
+-----------+----------+---------------+
10 rows in set (0.00 sec)
```
##### d.定义显示格式
​ CONCAT() 函数用于连接字符串
```shell
MySQL [company]> SELECT concat(name, 's annual salary: ', salary*14) AS Annual_salary FROM employee5;
+------------------------------------+
| Annual_salary |
+------------------------------------+
| jacks annual salary: 70000.00 |
| toms annual salary: 77000.00 |
| robins annual salary: 112000.00 |
| alices annual salary: 100800.00 |
| s annual salary: 8400.00 |
| harrys annual salary: 84000.00 |
| emmas annual salary: 280000.00 |
| christines annual salary: 30800.00 |
| zhuzhus annual salary: 30800.00 |
| gougous annual salary: 30800.00 |
+------------------------------------+
10 rows in set (0.00 sec)
```
##### e.单条件查询
```shell
MySQL [company]> SELECT name,post FROM employee5 WHERE post='hr';
+-------+------+
| name | post |
+-------+------+
| | hr |
| harry | hr |
+-------+------+
2 rows in set (0.00 sec)
```
##### f.多条件查询
```shell
MySQL [company]> SELECT name,salary FROM employee5 WHERE post='hr' AND salary>10000;
Empty set (0.00 sec)
MySQL [company]> select * from employee5 where salary>5000 and salary<10000 or dep_id=102;
+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
| 2 | tom | male | 2018-02-03 | instructor | teach | 5500.00 | 501 | 100 |
| 3 | robin | male | 2018-02-02 | instructor | teach | 8000.00 | 501 | 100 |
| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
| 6 | harry | male | 2018-02-02 | hr | NULL | 6000.00 | 502 | 101 |
| 7 | emma | female | 2018-02-06 | sale | salecc | 20000.00 | 503 | 102 |
| 8 | christine | female | 2018-02-05 | sale | salecc | 2200.00 | 503 | 102 |
| 9 | zhuzhu | male | 2018-02-05 | sale | NULL | 2200.00 | 503 | 102 |
| 10 | gougou | male | 2018-02-05 | sale | | 2200.00 | 503 | 102 |
+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
8 rows in set (0.00 sec)
```
##### g.关键字
​ BETWEEN AND
```shell
MySQL [company]> SELECT name,salary FROM employee5 WHERE salary BETWEEN 5000 AND 15000;
+-------+---------+
| name | salary |
+-------+---------+
| jack | 5000.00 |
| tom | 5500.00 |
| robin | 8000.00 |
| alice | 7200.00 |
| harry | 6000.00 |
+-------+---------+
5 rows in set (0.00 sec)
MySQL [company]> SELECT name,salary FROM employee5 WHERE salary NOT BETWEEN 5000 AND 15000;
+-----------+----------+
| name | salary |
+-----------+----------+
| | 600.00 |
| emma | 20000.00 |
| christine | 2200.00 |
| zhuzhu | 2200.00 |
| gougou | 2200.00 |
+-----------+----------+
5 rows in set (0.00 sec)
```
​ IS NULL
```shell
MySQL [company]> SELECT name,job_description FROM employee5 WHERE job_description IS NULL;
+--------+-----------------+
| name | job_description |
+--------+-----------------+
| harry | NULL |
| zhuzhu | NULL |
+--------+-----------------+
2 rows in set (0.00 sec)
MySQL [company]> SELECT name,job_description FROM employee5 WHERE job_description IS NOT NULL;
+-----------+-----------------+
| name | job_description |
+-----------+-----------------+
| jack | teach |
| tom | teach |
| robin | teach |
| alice | teach |
| | hrcc |
| emma | salecc |
| christine | salecc |
| gougou | |
+-----------+-----------------+
8 rows in set (0.00 sec)
MySQL [company]> SELECT name,job_description FROM employee5 WHERE job_description='';
+--------+-----------------+
| name | job_description |
+--------+-----------------+
| gougou | |
+--------+-----------------+
1 row in set (0.00 sec)
```
注意：（NULL说明）
​ 等价于没有任何值、是未知数
​ NULL与0、空字符串、空格都不同,NULL没有分配存储空间
​ 对空值做加、减、乘、除等运算操作，结果仍为空
​ 比较时使用关键字用“is null”和“is not null”
​ 排序时比其他数据都小（索引默认是降序排列，小→大），所以NULL值总是排在最前
IN集合查询
```shell
MySQL [company]> SELECT name, salary FROM employee5 WHERE salary=4000 OR salary=5000 OR salary=6000 OR salary=9000 ;
+-------+---------+
| name | salary |
+-------+---------+
| jack | 5000.00 |
| harry | 6000.00 |
+-------+---------+
2 rows in set (0.00 sec)
MySQL [company]> SELECT name, salary FROM employee5 WHERE salary IN (4000,5000,6000,9000) ;
+-------+---------+
| name | salary |
+-------+---------+
| jack | 5000.00 |
| harry | 6000.00 |
+-------+---------+
2 rows in set (0.00 sec)
MySQL [company]> SELECT name, salary FROM employee5 WHERE salary NOT IN (4000,5000,6000,9000) ;
+-----------+----------+
| name | salary |
+-----------+----------+
| tom | 5500.00 |
| robin | 8000.00 |
| alice | 7200.00 |
| | 600.00 |
| emma | 20000.00 |
| christine | 2200.00 |
| zhuzhu | 2200.00 |
| gougou | 2200.00 |
+-----------+----------+
8 rows in set (0.01 sec)
```
##### h.模糊查询
​ 关键字LIKE
​ 通配符%：所有字符
​ 通配符_： 一个字符
```shell
MySQL [company]> SELECT * FROM employee5 WHERE name LIKE 'al%';
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
1 row in set (0.00 sec)
MySQL [company]> SELECT * FROM employee5 WHERE name LIKE 'al___';
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
1 row in set (0.00 sec)
```
##### i.排序查询
```shell
MySQL [company]> select name,salary from employee5 order by salary;
+-----------+----------+
| name | salary |
+-----------+----------+
| | 600.00 |
| christine | 2200.00 |
| zhuzhu | 2200.00 |
| gougou | 2200.00 |
| jack | 5000.00 |
| tom | 5500.00 |
| harry | 6000.00 |
| alice | 7200.00 |
| robin | 8000.00 |
| emma | 20000.00 |
+-----------+----------+
10 rows in set (0.01 sec)
MySQL [company]> select name,salary from employee5 order by salary desc;
+-----------+----------+
| name | salary |
+-----------+----------+
| emma | 20000.00 |
| robin | 8000.00 |
| alice | 7200.00 |
| harry | 6000.00 |
| tom | 5500.00 |
| jack | 5000.00 |
| christine | 2200.00 |
| zhuzhu | 2200.00 |
| gougou | 2200.00 |
| | 600.00 |
+-----------+----------+
MySQL [company]> select name,salary from employee5 order by salary desc limit 3; //控制显示前3行
+-------+----------+
| name | salary |
+-------+----------+
| emma | 20000.00 |
| robin | 8000.00 |
| alice | 7200.00 |
+-------+----------+
3 rows in set (0.00 sec)
MySQL [company]> select name,salary from employee5 order by salary desc limit 1,3; //从序号1开始显示三行的内容
+-------+---------+
| name | salary |
+-------+---------+
| robin | 8000.00 |
| alice | 7200.00 |
| harry | 6000.00 |
+-------+---------+
3 rows in set (0.00 sec)
```
注意：
​ ascending 美音 /ə'sɛndɪŋ/ 升序
​ descending 美音 /dɪ'sɛndɪŋ/ 降序
##### j.集合函数查询
​ count：可以查看共有多少条记录
```shell
MySQL [company]> select count(*) from employee5;
+----------+
| count(*) |
+----------+
| 10 |
+----------+
1 row in set (0.00 sec)
MySQL [company]> select count(name) from employee5;
+-------------+
| count(name) |
+-------------+
| 10 |
+-------------+
1 row in set (0.00 sec)
```
​ max：查看最大值
```shell
MySQL [company]> select max(salary) from employee5;
+-------------+
| max(salary) |
+-------------+
| 20000.00 |
+-------------+
1 row in set (0.00 sec)
```
​ min：查看最小值
```shell
MySQL [company]> select min(salary) from employee5;
+-------------+
| min(salary) |
+-------------+
| 600.00 |
+-------------+
1 row in set (0.00 sec)
```
​ avg：查看平均值
```shell
MySQL [company]> select avg(salary) from employee5;
+-------------+
| avg(salary) |
+-------------+
| 5890.000000 |
+-------------+
1 row in set (0.00 sec)
```
​ sum：求和
​ sale这个部门的总工资
```shell
MySQL [company]> select concat("Total Department Wages：",sum(salary)) from employee5 where post='sale';
+-------------------------------------------------+
| concat("Total Department Wages：",sum(salary)) |
+-------------------------------------------------+
| Total Department Wages：26600.00 |
+-------------------------------------------------+
1 row in set (0.00 sec)
```
​ 获取薪水最高的这个人的详细信息
```shell
MySQL [company]> select * from employee5 where salary = (select max(salary) from employee5);
+----+------+--------+------------+------+-----------------+----------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+------+--------+------------+------+-----------------+----------+--------+--------+
| 7 | emma | female | 2018-02-06 | sale | salecc | 20000.00 | 503 | 102 |
+----+------+--------+------------+------+-----------------+----------+--------+--------+
1 row in set (0.00 sec)
```
##### k.分组查询
​ GROUP BY和GROUP_CONCAT()函数一起使用
​ 获取部门ID相同的员工并把名字拼接到一起
```shell
MySQL [company]> SELECT dep_id,GROUP_CONCAT(name) FROM employee5 GROUP BY dep_id;
+--------+------------------------------+
| dep_id | GROUP_CONCAT(name) |
+--------+------------------------------+
| 100 | jack,tom,robin,alice |
| 101 | ,harry |
| 102 | emma,christine,zhuzhu,gougou |
+--------+------------------------------+
3 rows in set (0.01 sec)
```
​ GROUP BY和集合函数一起使用
​ 获取部门最高薪资
```shell
MySQL [company]> SELECT post,max(salary) FROM employee5 GROUP BY post;
+------------+-------------+
| post | max(salary) |
+------------+-------------+
| hr | 6000.00 |
| instructor | 8000.00 |
| sale | 20000.00 |
+------------+-------------+
3 rows in set (0.00 sec)
```
##### l.正则查询
```shell
以什么开头
MySQL [company]> SELECT * FROM employee5 WHERE name REGEXP '^ali';
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
1 row in set (0.00 sec)
以什么结尾
MySQL [company]> SELECT * FROM employee5 WHERE name REGEXP 'ce$';
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
1 row in set (0.01 sec)
连续出现n次
MySQL [company]> SELECT * FROM employee5 WHERE name REGEXP 'm{2}';
+----+------+--------+------------+------+-----------------+----------+--------+--------+
| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
+----+------+--------+------------+------+-----------------+----------+--------+--------+
| 7 | emma | female | 2018-02-06 | sale | salecc | 20000.00 | 503 | 102 |
+----+------+--------+------------+------+-----------------+----------+--------+--------+
1 row in set (0.00 sec)
```
## 二：多表联合查询
【扩展了解】
#### 1.数据准备
##### 表：company.employee6
创建表：
```shell
MySQL [company]> create table employee6(
emp_id int auto_increment primary key not null,
emp_name varchar(50),
age int,
dept_id int);
Query OK, 0 rows affected (0.65 sec)
```
查看表结构：
```shell
MySQL [company]> desc employee6;
+----------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+----------------+
| emp_id | int(11) | NO | PRI | NULL | auto_increment |
| emp_name | varchar(50) | YES | | NULL | |
| age | int(11) | YES | | NULL | |
| dept_id | int(11) | YES | | NULL | |
+----------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
```
插入模拟数据：
```shell
MySQL [company]> insert into employee6(emp_name,age,dept_id) values
('',19,200),
('tom',26,201),
('jack',30,201),
('alice',24,202),
('robin',40,200),
('xingdian',16,200),
('natasha',28,204);
```
查看数据：
```shell
MySQL [company]> select * from employee6;
+--------+----------+------+---------+
| emp_id | emp_name | age | dept_id |
+--------+----------+------+---------+
| 1 | | 19 | 200 |
| 2 | tom | 26 | 201 |
| 3 | jack | 30 | 201 |
| 4 | alice | 24 | 202 |
| 5 | robin | 40 | 200 |
| 6 | xingdian | 16 | 200 |
| 7 | natasha | 28 | 204 |
+--------+----------+------+---------+
7 rows in set (0.00 sec)
```
##### 表：company.department6
创建表：
```shell
MySQL [company]> create table department6(
dept_id int,
dept_name varchar(100)
);
Query OK, 0 rows affected (0.33 sec)
```
查看表结构：
```shell
MySQL [company]> desc department6;
+-----------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+--------------+------+-----+---------+-------+
| dept_id | int(11) | YES | | NULL | |
| dept_name | varchar(100) | YES | | NULL | |
+-----------+--------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
```
模拟插入数据：
```shell
MySQL [company]> insert into department6 values
(200,'hr'),
(201,'it'),
(202,'sale'),
(203,'fd');
```
查看数据：
```shell
MySQL [company]> select * from department6;
+---------+-----------+
| dept_id | dept_name |
+---------+-----------+
| 200 | hr |
| 201 | it |
| 202 | sale |
| 203 | fd |
+---------+-----------+
4 rows in set (0.01 sec)
```
#### 2.多表的连接查询
​ 交叉连接：生成笛卡尔积，它不使用任何匹配条件；交叉联接返回左表中的所有行，左表中的每一行与右表中的所有行组合
​ 内连接：只连接匹配的行
​ 外连接
​ 左连接：会显示左边表内所有的值，不论在右边表内匹不匹配
​ 右连接：会显示右边表内所有的值，不论在左边表内匹不匹配
​ 全外连接：包含左、右两个表的全部行
##### 交叉连接
```shell
MySQL [company]> select employee6.emp_name,employee6.age,employee6.dept_id,department6.dept_name from employee6,department6;
+----------+------+---------+-----------+
| emp_name | age | dept_id | dept_name |
+----------+------+---------+-----------+
| | 19 | 200 | hr |
| | 19 | 200 | it |
| | 19 | 200 | sale |
| | 19 | 200 | fd |
| tom | 26 | 201 | hr |
| tom | 26 | 201 | it |
| tom | 26 | 201 | sale |
| tom | 26 | 201 | fd |
| jack | 30 | 201 | hr |
| jack | 30 | 201 | it |
| jack | 30 | 201 | sale |
| jack | 30 | 201 | fd |
| alice | 24 | 202 | hr |
| alice | 24 | 202 | it |
| alice | 24 | 202 | sale |
| alice | 24 | 202 | fd |
| robin | 40 | 200 | hr |
| robin | 40 | 200 | it |
| robin | 40 | 200 | sale |
| robin | 40 | 200 | fd |
| xingdian | 16 | 200 | hr |
| xingdian | 16 | 200 | it |
| xingdian | 16 | 200 | sale |
| xingdian | 16 | 200 | fd |
| natasha | 28 | 204 | hr |
| natasha | 28 | 204 | it |
| natasha | 28 | 204 | sale |
| natasha | 28 | 204 | fd |
+----------+------+---------+-----------+
28 rows in set (0.00 sec)
```
##### 内连接
​ 获取有部门的员工 (部门表中没有natasha所在的部门)
```shell
MySQL [company]> select employee6.emp_name,employee6.age,employee6.dept_id,department6.dept_name from employee6,department6 where employee6.dept_id=department6.dept_id;
+----------+------+---------+-----------+
| emp_name | age | dept_id | dept_name |
+----------+------+---------+-----------+
| | 19 | 200 | hr |
| tom | 26 | 201 | it |
| jack | 30 | 201 | it |
| alice | 24 | 202 | sale |
| robin | 40 | 200 | hr |
| xingdian | 16 | 200 | hr |
+----------+------+---------+-----------+
6 rows in set (0.00 sec)
MySQL [company]> select employee6.emp_name,department6.dept_name from employee6 inner join department6 on employee6.dept_id=department6.dept_id;
+----------+-----------+
| emp_name | dept_name |
+----------+-----------+
| | hr |
| tom | it |
| jack | it |
| alice | sale |
| robin | hr |
| xingdian | hr |
+----------+-----------+
6 rows in set (0.01 sec)
```
##### 外连接
语法：
​ SELECT 字段列表 FROM 表1 LEFT|RIGHT JOIN 表2 ON 表1.字段 = 表2.字段;
注意：
​ 先用谁谁就是左
###### 左连接 left join
```shell
找出所有员工及所属的部门，包括没有部门的员工
MySQL [company]> select emp_id,emp_name,dept_name from employee6 left join department6 on employee6.dept_id = department6.dept_id;
+--------+----------+-----------+
| emp_id | emp_name | dept_name |
+--------+----------+-----------+
| 1 | | hr |
| 5 | robin | hr |
| 6 | xingdian | hr |
| 2 | tom | it |
| 3 | jack | it |
| 4 | alice | sale |
| 7 | natasha | NULL |
+--------+----------+-----------+
7 rows in set (0.00 sec)
```
###### 右连接right join
```
找出所有部门包含的员工，包括空部门
MySQL [company]> select emp_id,emp_name,dept_name from employee6 right join department6 on employee6.dept_id = department6.dept_id;
+--------+----------+-----------+
| emp_id | emp_name | dept_name |
+--------+----------+-----------+
| 1 | | hr |
| 2 | tom | it |
| 3 | jack | it |
| 4 | alice | sale |
| 5 | robin | hr |
| 6 | xingdian | hr |
| NULL | NULL | fd |
+--------+----------+-----------+
7 rows in set (0.00 sec)
```
##### 全外连接
```shell
MySQL [company]> select * from employee6 full join department6;
+--------+----------+------+---------+---------+-----------+
| emp_id | emp_name | age | dept_id | dept_id | dept_name |
+--------+----------+------+---------+---------+-----------+
| 1 | | 19 | 200 | 200 | hr |
| 1 | | 19 | 200 | 201 | it |
| 1 | | 19 | 200 | 202 | sale |
| 1 | | 19 | 200 | 203 | fd |
| 2 | tom | 26 | 201 | 200 | hr |
| 2 | tom | 26 | 201 | 201 | it |
| 2 | tom | 26 | 201 | 202 | sale |
| 2 | tom | 26 | 201 | 203 | fd |
| 3 | jack | 30 | 201 | 200 | hr |
| 3 | jack | 30 | 201 | 201 | it |
| 3 | jack | 30 | 201 | 202 | sale |
| 3 | jack | 30 | 201 | 203 | fd |
| 4 | alice | 24 | 202 | 200 | hr |
| 4 | alice | 24 | 202 | 201 | it |
| 4 | alice | 24 | 202 | 202 | sale |
| 4 | alice | 24 | 202 | 203 | fd |
| 5 | robin | 40 | 200 | 200 | hr |
| 5 | robin | 40 | 200 | 201 | it |
| 5 | robin | 40 | 200 | 202 | sale |
| 5 | robin | 40 | 200 | 203 | fd |
| 6 | xingdian | 16 | 200 | 200 | hr |
| 6 | xingdian | 16 | 200 | 201 | it |
| 6 | xingdian | 16 | 200 | 202 | sale |
| 6 | xingdian | 16 | 200 | 203 | fd |
| 7 | natasha | 28 | 204 | 200 | hr |
| 7 | natasha | 28 | 204 | 201 | it |
| 7 | natasha | 28 | 204 | 202 | sale |
| 7 | natasha | 28 | 204 | 203 | fd |
+--------+----------+------+---------+---------+-----------+
28 rows in set (0.00 sec)
```
#### 3.复合条件连接查询
##### 案例一
​ 找出公司所有部门中年龄大于25岁的员工
​ 以内连接的方式查询employee6和department6表，并且employee6表中的age字段值必须大于25
```shell
MySQL [company]> select emp_id,emp_name,dept_name FROM employee6,department6 WHERE employee6.dept_id = department6.dept_id AND age > 25;
+--------+----------+-----------+
| emp_id | emp_name | dept_name |
+--------+----------+-----------+
| 5 | robin | hr |
| 2 | tom | it |
| 3 | jack | it |
+--------+----------+-----------+
3 rows in set (0.01 sec)
```
##### 案例二
​ 以内连接的方式查询employee6和department6表，并且以age字段的升序方式显示
```shell
MySQL [company]> select emp_id,emp_name,dept_name FROM employee6,department6 WHERE employee6.dept_id = department6.dept_id ORDER BY age asc;
+--------+----------+-----------+
| emp_id | emp_name | dept_name |
+--------+----------+-----------+
| 6 | xingdian | hr |
| 1 | | hr |
| 4 | alice | sale |
| 2 | tom | it |
| 3 | jack | it |
| 5 | robin | hr |
+--------+----------+-----------+
```
#### 4.子查询
​ 子查询是将一个查询语句嵌套在另一个查询语句中
​ 内层查询语句的查询结果，可以为外层查询语句提供查询条件
​ 子查询中可以包含：IN、NOT IN等关键字；还可以包含比较运算符：= 、 !=、> 、<等
##### 案例一
​ 带IN关键字的子查询；查询employee表，但dept_id必须在department表中出现过
```shell
MySQL [company]> select * from employee6 WHERE dept_id IN (select dept_id FROM department6);
+--------+----------+------+---------+
| emp_id | emp_name | age | dept_id |
+--------+----------+------+---------+
| 1 | | 19 | 200 |
| 2 | tom | 26 | 201 |
| 3 | jack | 30 | 201 |
| 4 | alice | 24 | 202 |
| 5 | robin | 40 | 200 |
| 6 | xingdian | 16 | 200 |
+--------+----------+------+---------+
6 rows in set (0.00 sec)
```
##### 案例二
​ 带比较运算符的子查询；查询年龄大于等于25岁员工所在部门（查询老龄化的部门）
```shell
MySQL [company]> select dept_id,dept_name FROM department6 WHERE dept_id IN (SELECT DISTINCT dept_id FROM employee6 WHERE age >=25);
+---------+-----------+
| dept_id | dept_name |
+---------+-----------+
| 201 | it |
| 200 | hr |
+---------+-----------+
2 rows in set (0.00 sec)
```

861
第二章：数据库管理及数据类型.md Normal file
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<h1><center>数据库管理及数据类型</center></h1>
------
## 一：数据类型
#### 1.数值类型
##### 整数类型
​ 整数类型：TINYINT SMALLINT MEDIUMINT INT BIGINT
​ 作用：用于存储用户的年龄、游戏的Level、经验值等
![image-20220920124821420](https://xingdian-image.oss-cn-beijing.aliyuncs.com/xingdian-image/image-20220920124821420.png)
##### 浮点数类型
​ 浮点数类型：FLOAT DOUBLE
​ 作用：用于存储用户的身高、体重、薪水等
![image-20220920124919373](https://xingdian-image.oss-cn-beijing.aliyuncs.com/xingdian-image/image-20220920124919373.png)
```shell
float(5,3) 5宽度 3精度
注意：
宽度不算小数点
宽度-精度=点前
案例：
MySQL [(none)]> create database diandian;
Query OK, 1 row affected (0.00 sec)
MySQL [(none)]> use diandian
Database changed
MySQL [diandian]> create table t1(id float(6,2));
Query OK, 0 rows affected (0.24 sec)
MySQL [diandian]> insert into t1 values('2.22');
```
##### 定点数类型
​ 定点数类型：DEC
​ 定点数在MySQL内部以字符串形式存储，比浮点数更精确，适合用来表示货币等精度高的数据
##### 位类型
​ 位类型：BIT
​ BIT(M)可以用来存放多位二进制数，M范围从1~64，如果不写默认为1位
#### 2.字符串类型
​ CHAR系列 CHAR VARCHAR
​ TEXT系列 TINYTEXT TEXT MEDIUMTEXT LONGTEXT
​ BLOB 系列 TINYBLOB BLOB MEDIUMBLOB LONGBLOB
​ BINARY系列 BINARY VARBINARY
![image-20220920132114919](https://xingdian-image.oss-cn-beijing.aliyuncs.com/xingdian-image/image-20220920132114919.png)
##### 枚举类型
​ 枚举类型：枚举列可以把一些不重复的字符串存储成一个预定义的集合
```shell
mysql> create table enum_table( e ENUM('fish','apple','dog'));
Query OK, 0 rows affected (0.35 sec)
mysql> insert into enum_table(e) values('fish');
Query OK, 1 row affected (0.11 sec)
mysql> select * from enum_table;
+------+
| e |
+------+
| fish |
+------+
1 row in set (0.00 sec)
mysql> insert into enum_table(e) values('nihao');
ERROR 1265 (01000): Data truncated for column 'e' at row 1
```
##### 时间和日期类型
​ 时间和日期类型：DATE TIME DATETIME TIMESTAMP YEAR
​ 作用：用于存储用户的注册时间，文章的发布时间，文章的更新时间，员工的入职时间等
![image-20220920132630856](https://xingdian-image.oss-cn-beijing.aliyuncs.com/xingdian-image/image-20220920132630856.png)
```shell
mysql> create table t8 (
id1 timestamp NOT NULL default CURRENT_TIMESTAMP,
id2 datetime default NULL
);
timestamp 类型的列还有个特性：默认情况下，在 insert, update 数据时，timestamp 列会自动以当前时间（CURRENT_TIMESTAMP）填充/更新。“自动”的意思就是，你不去管它，MySQL 会替你去处理。
mysql> insert into t8(id1) values('20180109000000');
mysql> select * from t8;
+---------------------+------+
| id1 | d2 |
+---------------------+------+
| 2018-01-09 00:00:00 | NULL |
+---------------------+------+
1 row in set (0.00 sec)
扩展：
select now();查看当前时间
```
## 二：表操作
#### 1.案例
表:：school.student1
```
字段 字段 字段
id name sex age
1 tom male 23 记录
2 jack male 21 记录
3 alice female 19 记录
```
语法：
```shell
create table 表名(自定义)(
字段名1 类型[(宽度) 约束条件],
字段名2 类型[(宽度) 约束条件],
字段名3 类型[(宽度) 约束条件]
)[存储引擎 字符集];
==在同一张表中，字段名是不能相同
==宽度和约束条件可选
==字段名和类型是必须的
```
创建库表：
```shell
mysql> CREATE DATABASE school; //创建数据库school
mysql> use school;
mysql> create table student1(
id int,
name varchar(50),
sex enum('m','f'),
age int
);
Query OK, 0 rows affected (0.03 sec)
```
查看库：
```shell
mysql> show tables;
+------------------+
| Tables_in_school |
+------------------+
| student1 |
+------------------+
1 row in set (0.00 sec)
```
插入语法：
```
insert into 表名(字段1,字段2...) values(字段值列表...);
```
插入数据：
```shell
mysql> insert into student1(id,name,sex,age) values(1,'xingdia','m','26');
```
查看表结构：
```shell
mysql> desc student1;
+-------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(50) | YES | | NULL | |
| sex | enum('m','f') | YES | | NULL | |
| age | int(11) | YES | | NULL | |
+-------+---------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
```
查询数据：
```shell
mysql> select id,name,sex,age from student1; //查询表中所有字段的值
Empty set (0.00 sec)
mysql> select * from student1; /查询表中所有字段的值
Empty set (0.00 sec)
mysql> select name,age from student1; //查询表中指定字段的值
Empty set (0.00 sec)
```
扩展插入：
```shell
mysql> insert into student1 values (1,'xingdian','m',33),(2,'alice','m',20),(3,'jack','m',40); //顺序插入
Query OK, 3 rows affected (0.14 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> insert into student1(name,age) values ('zhuzhu',10),('gougou',20); //只向指定的字段插入值
Query OK, 2 rows affected (0.12 sec)
Records: 2 Duplicates: 0 Warnings: 0
```
#### 2.案例
表：school.student2
```shell
字段名 数据类型
编号 id int
姓名 name varchar(50)
出生年份 born_year year
生日 birthday date
上课时间 class_time time
注册时间 reg_time datetime
```
创建表：
```shell
mysql> create table student2(
id int,
name varchar(50),
born_year year,
birthday date,
class_time time,
reg_time datetime
);
```
插入数据：
```shell
mysql> insert into student2 values(1,'tom',now(),now(),now(),now());
mysql> insert into student2 values(2,'jack',1982,19821120,123000,20140415162545);
```
表：school.student3
```shell
id id int
姓名 name varchar(50)
性别 sex enum('male','female')
爱好 hobby set('music','book','game','disc')
```
创建表：
```shell
mysql> create table student3(
id int,
name varchar(50),
sex enum('male','female'),
hobby set('music','book','game','disc')
);
```
查看表结构：
```shell
mysql> desc student3;
mysql> show create table student3\G
```
插入数据：
```shell
mysql> insert into student3 values (1,'tom','male','book,game');
mysql> insert into student3 values (2,'jack','male','film');
```
注意：
​ DESCRIBE查看表结构
```shell
DESCRIBE 表名;
DESC 表名;
```
​ 查看表详细结构
```shell
SHOW CREATE TABLE 表名;
```
## 三：表完整性约束
#### 1.作用
​ 用于保证数据的完整性和一致性
#### 2.约束条件
PRIMARY KEY (PK) 标识该字段为该表的主键，可以唯一的标识记录，不可以为空 UNIQUE + NOT NULL
FOREIGN KEY (FK) 标识该字段为该表的外键，实现表与表（父表主键/子表1外键/子表2外键）之间的关联
NOT NULL 标识该字段不能为空
UNIQUE KEY (UK) 标识该字段的值是唯一的，可以为空，一个表中可以有多个UNIQUE KEY
AUTO_INCREMENT 标识该字段的值自动增长（整数类型，而且为主键）
DEFAULT 为该字段设置默认值
注意：
​ 是否允许为空，默认NULL，可设置NOT NULL，字段不允许为空，必须赋值
​ 字段是否有默认值，缺省的默认值是NULL，如果插入记录时不给字段赋值，此字段使用默认值
```shell
MySQL [(none)]> sex enum('male','female') not null default 'male'
MySQL [(none)]> age int unsigned NOT NULL default 20 必须为正值（无符号） 不允许为空 默认是20
```
​ 是否是key 主键 primary key 外键 forengn key
#### 3.NOT NULL
表：school.student4
创建表：（注意前提需要有库）
```shell
mysql> create table school.student4(
id int not null,
name varchar(50) not null,
sex enum('m','f') default 'm' not null,
age int unsigned default 18 not null,
hobby set('music','disc','dance','book') default 'book,dance'
);
```
插入数据：（注意观察查询到的数据）
```shell
MySQL [(none)]> insert into school.student4(id,name) values(2,'robin');
Query OK, 1 row affected (0.08 sec)
MySQL [(none)]> select * from school.student4;
+----+-------+-----+-----+------------+
| id | name | sex | age | hobby |
+----+-------+-----+-----+------------+
| 2 | robin | m | 18 | dance,book |
+----+-------+-----+-----+------------+
1 row in set (0.00 sec)
```
注意报错的原因：
```shell
MySQL [(none)]> insert into school.student4 values(3,NULL,'m',40,'book');
ERROR 1048 (23000): Column 'name' cannot be null
```
#### 4.唯一约束
作用：
​ MySQL索引的建立对于MySQL的高效运行是很重要的，索引可以大大提高MySQL的检索速度
表：company.department1
创建表：
```shell
MySQL [(none)]> create database company;
Query OK, 1 row affected (0.01 sec)
MySQL [(none)]> CREATE TABLE company.department1 (dept_id INT,dept_name VARCHAR(30) UNIQUE,comment VARCHAR(50));
Query OK, 0 rows affected (0.34 sec)
```
查看表结构：
```shell
MySQL [(none)]> desc company.department1;
+-----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+-------------+------+-----+---------+-------+
| dept_id | int(11) | YES | | NULL | |
| dept_name | varchar(30) | YES | UNI | NULL | |
| comment | varchar(50) | YES | | NULL | |
+-----------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
```
数据插入：（注意查看插入数据时的提示）
```shell
MySQL [(none)]> insert into company.department1 values ("1","xingdian","hr");
Query OK, 1 row affected (0.04 sec)
MySQL [(none)]> insert into company.department1 values ("1","xingdian","hr");
ERROR 1062 (23000): Duplicate entry 'xingdian' for key 'dept_name'
```
#### 5.主键约束
注意：primary key 字段的值是不允许重复，且不允许不NULL（UNIQUE + NOT NULL）
表：school.student6
创建表：
```shell
MySQL [(none)]> create table school.student6(
id int primary key not null auto_increment,
name varchar(50) not null,
sex enum('male','female') not null default 'male',
age int not null default 18
);
Query OK, 0 rows affected (0.47 sec)
```
插入数据：
```shell
MySQL [(none)]> insert into school.student6 values (1,'alice','female',22);
Query OK, 1 row affected (0.18 sec)
MySQL [(none)]> insert into school.student6(name,sex,age) values
-> ('jack','male',19),
-> ('tom','male',23);
Query OK, 2 rows affected (0.14 sec)
Records: 2 Duplicates: 0 Warnings: 0
MySQL [(none)]> select * from school.student6;
+----+-------+--------+-----+
| id | name | sex | age |
+----+-------+--------+-----+
| 1 | alice | female | 22 |
| 2 | jack | male | 19 |
| 3 | tom | male | 23 |
+----+-------+--------+-----+
3 rows in set (0.00 sec)
```
## 四：修改表
语法格式：
修改表名
​ ALTER TABLE 表名 RENAME 新表名;
增加字段
​ ALTER TABLE 表名
​ ADD 字段名 数据类型 [完整性约束条件…],
​ ADD 字段名 数据类型 [完整性约束条件…];
​ ALTER TABLE 表名
​ ADD 字段名 数据类型 [完整性约束条件…] AFTER 字段名;
删除字段
​ ALTER TABLE 表名 DROP 字段名;
修改字段
​ ALTER TABLE 表名 MODIFY 字段名 数据类型 [完整性约束条件…];
​ ALTER TABLE 表名 CHANGE 旧字段名 新字段名 旧数据类型 [完整性约束条件…];
​ ALTER TABLE 表名 CHANGE 旧字段名 新字段名 新数据类型 [完整性约束条件…];
#### 1.修改数据库引擎
```shell
mysql> alter table service engine=innodb; //engine=myisam|memory|....
```
#### 2.添加字段
```shell
mysql> create table student10 (id int);
mysql> alter table student10 add name varchar(20) not null, add age int not null default 22;
mysql> alter table student10 add stu_num int not null after name; //添加name字段之后
mysql> alter table student10 add sex enum('male','female') default 'male' first; //添加到最前面
```
#### 3.删除字段
```shell
mysql> alter table student10 drop sex;
```
#### 4.修改字段类型
```shell
MySQL [school]> desc student10;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| age | int(11) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.01 sec)
MySQL [school]> alter table student10 modify age tinyint not null ;
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0
MySQL [school]> desc student10;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| age | tinyint(4) | NO | | NULL | |
+-------+------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
MySQL [school]> alter table student10 modify id int not null primary key ; ////修改字段类型、约束、主键
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
MySQL [school]> desc student10;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| age | tinyint(4) | NO | | NULL | |
+-------+------------+------+-----+---------+-------+
```
#### 5.增加约束
```shell
MySQL [school]> alter table student10 modify id int not null primary key ;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
MySQL [school]> desc student10;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| age | tinyint(4) | NO | | NULL | |
+-------+------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
MySQL [school]> alter table student10 modify id int not null primary key auto_increment;
ERROR 1068 (42000): Multiple primary key defined //错误，该字段已经是primary key
MySQL [school]> alter table student10 modify id int not null auto_increment;
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0
MySQL [school]> desc student10;
+-------+------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| age | tinyint(4) | NO | | NULL | |
+-------+------------+------+-----+---------+----------------+
```
#### 6.增加主键
```shell
MySQL [school]> desc student1;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| age | int(11) | YES | | NULL | |
| name | char(1) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
3 rows in set (0.00 sec)
MySQL [school]> alter table student1 add primary key(id);
MySQL [school]> desc student1;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| age | int(11) | YES | | NULL | |
| name | char(1) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
3 rows in set (0.00 sec)
```
#### 7.修改主键和自增
```shell
MySQL [school]> alter table student1 modify id int auto_increment;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
MySQL [school]> desc student1;
+-------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| age | int(11) | YES | | NULL | |
| name | char(1) | YES | | NULL | |
+-------+---------+------+-----+---------+----------------+
```
#### 8.删除主键
```shell
MySQL [school]> desc student10;
+-------+------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| age | tinyint(4) | NO | | NULL | |
+-------+------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
MySQL [school]> alter table student10 drop primary key;
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key
删除自增
ySQL [school]> alter table student10 modify id int not null;
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0
MySQL [school]> desc student10;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| age | tinyint(4) | NO | | NULL | |
+-------+------------+------+-----+---------+-------+
MySQL [school]> alter table student10 drop primary key;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
MySQL [school]> desc student10;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id | int(11) | NO | | NULL | |
| age | tinyint(4) | NO | | NULL | |
+-------+------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
```
#### 9.复制表
复制表结构＋记录 （key不会复制: 主键、外键和索引）复制表结构/记录+表结构，不会将Key复制
```shell
mysql> create table new_service select * from service;
```
只复制表结构
```shell
mysql> create table new1_service select * from service where 1=2; //条件为假，查不到任何记录
```
可以复制主键,只复制表结构
```shell
mysql> create table t4 like employees;
```
#### 10.删除表
```shell
mysql> DROP TABLE 表名;
```
#### 11.修改数据表中字段的值
语法：
​ Update 表名 set 列名=值where 条件
```shell
mysql> update student set name='123' where id=1;
```
删除某一行：
​ delete from 表名 where id=1
```shell
mysql> delete from type where id=1;
```
## 五：库操作
#### 1.简介
​ 系统自带库的含义及作用
```shell
MySQL [(none)]> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
+--------------------+
```
information_schema：虚拟库，主要存储了系统中的一些数据库对象的信息，例如用户表信息、列信息、权限信息、字符信息等
performance_schema：主要存储数据库服务器的性能参数
mysql：授权库，主要存储系统用户的权限信息
sys：主要存储数据库服务器的性能参数
注意：information_schema
​ SCHEMATA 存放的是系统中的库
```shell
MySQL [information_schema]> select * from information_schema.SCHEMATA;
+--------------+--------------------+----------------------------+------------------------+----------+
| CATALOG_NAME | SCHEMA_NAME | DEFAULT_CHARACTER_SET_NAME | DEFAULT_COLLATION_NAME | SQL_PATH |
+--------------+--------------------+----------------------------+------------------------+----------+
| def | information_schema | utf8 | utf8_general_ci | NULL |
| def | mysql | latin1 | latin1_swedish_ci | NULL |
| def | performance_schema | utf8 | utf8_general_ci | NULL |
| def | school | latin1 | latin1_swedish_ci | NULL |
| def | sys | utf8 | utf8_general_ci | NULL |
+--------------+--------------------+----------------------------+------------------------+----------+
目录_名称
实际库_名称
默认_字符_设置_名称
默认_分类_名称
```
​ TABLES 存储表名
```shell
MySQL [information_schema]> select * from information_schema.TABLES\G
*************************** 283. row ***************************
TABLE_CATALOG: def
TABLE_SCHEMA: xingdian
TABLE_NAME: t1
TABLE_TYPE: BASE TABLE
ENGINE: InnoDB
VERSION: 10
ROW_FORMAT: Dynamic
TABLE_ROWS: 1
AVG_ROW_LENGTH: 16384
DATA_LENGTH: 16384
MAX_DATA_LENGTH: 0
INDEX_LENGTH: 0
DATA_FREE: 0
AUTO_INCREMENT: NULL
CREATE_TIME: 2022-09-22 08:18:38
UPDATE_TIME: 2022-09-22 08:18:54
CHECK_TIME: NULL
TABLE_COLLATION: latin1_swedish_ci
CHECKSUM: NULL
CREATE_OPTIONS:
TABLE_COMMENT:
283 rows in set (0.02 sec)
```
​ COLUMNS 存储字段
```shell
*************************** 3083. row ***************************
TABLE_CATALOG: def
TABLE_SCHEMA: xingdian
TABLE_NAME: t1
COLUMN_NAME: id
ORDINAL_POSITION: 1
COLUMN_DEFAULT: NULL
IS_NULLABLE: YES
DATA_TYPE: int
CHARACTER_MAXIMUM_LENGTH: NULL
CHARACTER_OCTET_LENGTH: NULL
NUMERIC_PRECISION: 10
NUMERIC_SCALE: 0
DATETIME_PRECISION: NULL
CHARACTER_SET_NAME: NULL
COLLATION_NAME: NULL
COLUMN_TYPE: int(11)
COLUMN_KEY:
EXTRA:
PRIVILEGES: select,insert,update,references
COLUMN_COMMENT:
GENERATION_EXPRESSION:
3083 rows in set (0.03 sec)
```
#### 2.创建库
方案一：交互式操作
```shell
mysql> create database xingdian;
```
数据库命名规则：
​ 区分大小写
​ 唯一性
​ 不能使用关键字如 create select
​ 不能单独使用数字
方案二：非交互式
```shell
mysql -u root -pQianFeng@123 -e "create database diandian"
```
#### 3.查看数据库
```shell
mysql> show databases;
mysql> show create database xingdian;
mysql> select database(); 查看当前所在的库
```
#### 4.切换数据库
```shell
mysql> use xingdian;
```
#### 5.删除数据库
```shell
mysql> DROP DATABASE 数据库名;
```

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<h1><center>数据库日志管理</center></h1>
------
## 一：日志管理
<img src="https://xingdian-image.oss-cn-beijing.aliyuncs.com/xingdian-image/image-20220925214046253.png" alt="image-20220925214046253" style="zoom:50%;" />
#### 1.日志分类
​ 错误日志 ：启动，停止，关闭失败报错。rpm安装日志位置 /var/log/mysqld.log
​ 通用查询日志：所有的查询都记下来
​ 二进制日志：实现备份，增量备份。只记录改变数据，除了select都记
​ 中继日志：读取主服务器的binlog，在本地回放。保持一致
​ slow log：慢查询日志，指导调优，定义某一个查询语句，定义超时时间，通过日志提供调优建议给开发人员
​ DDL log： 定义语句的日志
#### 2.Error Log
```shell
log-error=/var/log/mysqld.log
```
#### 3.Binary Log
```
log-bin=/var/log/mysql-bin/slave2
server-id=2
[root@slave2 ~]# mkdir /var/log/mysql-bin
[root@slave2 ~]# chown mysql.mysql /var/log/mysql-bin/
[root@slave2 ~]# systemctl restart mysqld
```
注意：
​ 需要提前开启
查看binlog日志：
```shel
[root@slave2 ~]# mysqlbinlog slave2-bin.000001 -v --base64-output=decode-rows
时间点 ： 141126 14:04:49
位置点 : at 106
注：
1. 重启mysqld 会截断
2. flush logs 会截断
3. reset master 删除所有binlog rm -rf /var/lib/mysql/*.000001
4. 删除部分
PURGE BINARY LOGS TO 'mysql-bin.010';
PURGE BINARY LOGS BEFORE '2019-04-02 22:46:26';
截取binlog
all：
# mysqlbinlog mysql.000002
datetime：
# mysqlbinlog mysql.000002 --start-datetime="2018-12-05 10:02:56"
# mysqlbinlog mysql.000002 --stop-datetime="2018-12-05 11:02:54"
# mysqlbinlog mysql.000002 --start-datetime="2018-12-05 10:02:56" --stop-datetime="2018-12-05 11:02:54"
position：
# mysqlbinlog mysql.000002 --start-position=260
# mysqlbinlog mysql.000002 --stop-position=260
# mysqlbinlog mysql.000002 --start-position=260 --stop-position=930
```
#### 4.Slow Query Log
开启慢查询日志：
```shell
[root@xingdian ~]# vim /etc/my.cnf
slow_query_log=1
slow_query_log_file=/var/log/mysql-slow/slow.log
long_query_time=3 设置慢查询超时时间 单位是：秒
```
创建对应目录：
```shell
[root@xingdian ~]# mkdir /var/log/mysql-slow
[root@xingdian ~]# chown mysql.mysql mysql-slow
```
重启服务：
```shell
[root@xingdian ~]# systemctl restart mysqld
```
验证：
```shell
[root@xingdian ~]# mysql -uroot -pQianFeng@123
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.7.39-log MySQL Community Server (GPL)
Copyright (c) 2000, 2022, 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> select sleep(6);
+----------+
| sleep(6) |
+----------+
| 0 |
+----------+
1 row in set (6.00 sec)
mysql> exit
Bye
[root@xingdian ~]# cat /var/log/mysql-slow/slow.log
/usr/sbin/mysqld, Version: 5.7.39-log (MySQL Community Server (GPL)). started with:
Tcp port: 0 Unix socket: /var/lib/mysql/mysql.sock
Time Id Command Argument
# Time: 2022-09-25T06:58:05.496205Z
# User@Host: root[root] @ localhost [] Id: 2
# Query_time: 6.007094 Lock_time: 0.000000 Rows_sent: 1 Rows_examined: 0
SET timestamp=1664089085;
select sleep(6);
```