MySQL基本操做

时间 2019-11-11

标签 mysql 基本栏目 MySQL 繁體版

原文原文链接

#显示全部的数据库mysql

MariaDB [(none)]> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| db                 |
| db_utf             |
| mysql              |
| performance_schema |
| test               |
+--------------------+

#建立数据库sql

MariaDB [(none)]> create database s18;
Query OK, 1 row affected (0.00 sec)

MariaDB [(none)]> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| db                 |
| db_utf             |
| mysql              |
| performance_schema |
| s18                |
| test               |
+--------------------+

#查看数据库的建立信息数据库

MariaDB [(none)]> show create database s18;
+----------+----------------------------------------------------------------+
| Database | Create Database                                                |
+----------+----------------------------------------------------------------+
| s18      | CREATE DATABASE `s18` /*!40100 DEFAULT CHARACTER SET latin1 */ |
+----------+----------------------------------------------------------------+

MariaDB [(none)]> create database s18_2 character set utf8;
Query OK, 1 row affected (0.00 sec)

MariaDB [(none)]> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| db                 |
| db_utf             |
| mysql              |
| performance_schema |
| s18                |
| s18_2              |
| test               |
+--------------------+

MariaDB [(none)]> show create database s18_2;
+----------+----------------------------------------------------------------+
| Database | Create Database                                                |
+----------+----------------------------------------------------------------+
| s18_2    | CREATE DATABASE `s18_2` /*!40100 DEFAULT CHARACTER SET utf8 */ |
+----------+----------------------------------------------------------------+

#修改数据库字符集编码ide

MariaDB [(none)]> alter database s18 character set utf8;
Query OK, 1 row affected (0.01 sec)

#删除数据库函数

MariaDB [(none)]> drop database s18_2;
Query OK, 0 rows affected (0.00 sec)

#使用数据库编码

MariaDB [(none)]> use s18;
Database changed

#查看当前使用的数据库spa

MariaDB [s18]> select database();
+------------+
| database() |
+------------+
| s18        |
+------------+

数据表操做code

#建立表orm

MariaDB [s18]> create table c1(
    ->     name varchar(20),
    ->     age int);
Query OK, 0 rows affected (0.01 sec)

#查看表结构blog

MariaDB [s18]> desc c1;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| name  | varchar(20) | YES  |     | NULL    |       |
| age   | int(11)     | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

#查看全部字段

MariaDB [s18]> select * from c1;
Empty set (0.00 sec)

#查看当前数据库下的全部表

MariaDB [s18]> show tables;
+---------------+
| Tables_in_s18 |
+---------------+
| c1            |
+---------------+

#查看数据库的建立表信息

MariaDB [s18]> show create table c1\G
*************************** 1. row ***************************
       Table: c1
Create Table: CREATE TABLE `c1` (
  `name` varchar(20) DEFAULT NULL,
  `age` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

#添加新字段

MariaDB [s18]> alter table c1 add salary float NOT NULL;
Query OK, 0 rows affected (0.01 sec)               

MariaDB [s18]> desc c1;
+--------+-------------+------+-----+---------+-------+
| Field  | Type        | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| name   | varchar(20) | YES  |     | NULL    |       |
| age    | int(11)     | YES  |     | NULL    |       |
| salary | float       | NO   |     | NULL    |       |
+--------+-------------+------+-----+---------+-------+

完整性约束

1.主键约束

primary key:非空(not null)且惟一(unique)

MariaDB [s18]> alter table c1 add id int first;
Query OK, 0 rows affected (0.01 sec)       

MariaDB [s18]> desc c1;
+--------+-------------+------+-----+---------+-------+
| Field  | Type        | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id     | int(11)     | YES  |     | NULL    |       |
| name   | varchar(20) | YES  |     | NULL    |       |
| age    | int(11)     | YES  |     | NULL    |       |
| salary | float       | NO   |     | NULL    |       |
+--------+-------------+------+-----+---------+-------+

#修改字段

MariaDB [s18]> alter table c1 modify id int primary key auto_increment first;
Query OK, 0 rows affected (0.01 sec) 

MariaDB [s18]> desc c1;
+--------+-------------+------+-----+---------+----------------+
| Field  | Type        | Null | Key | Default | Extra          |
+--------+-------------+------+-----+---------+----------------+
| id     | int(11)     | NO   | PRI | NULL    | auto_increment |
| name   | varchar(20) | YES  |     | NULL    |                |
| age    | int(11)     | YES  |     | NULL    |                |
| salary | float       | NO   |     | NULL    |                |
+--------+-------------+------+-----+---------+----------------+

#更改表名

MariaDB [s18]> alter table c1 change name names varchar(20);
Query OK, 0 rows affected (0.01 sec)  

MariaDB [s18]> desc c1;
+--------+-------------+------+-----+---------+----------------+
| Field  | Type        | Null | Key | Default | Extra          |
+--------+-------------+------+-----+---------+----------------+
| id     | int(11)     | NO   | PRI | NULL    | auto_increment |
| names  | varchar(20) | YES  |     | NULL    |                |
| age    | int(11)     | YES  |     | NULL    |                |
| salary | float       | NO   |     | NULL    |                |
+--------+-------------+------+-----+---------+----------------+

#删除表字段

MariaDB [s18]> alter table c1 drop names;
Query OK, 0 rows affected (0.01 sec) 

MariaDB [s18]> desc c1;
+--------+---------+------+-----+---------+----------------+
| Field  | Type    | Null | Key | Default | Extra          |
+--------+---------+------+-----+---------+----------------+
| id     | int(11) | NO   | PRI | NULL    | auto_increment |
| age    | int(11) | YES  |     | NULL    |                |
| salary | float   | NO   |     | NULL    |                |
+--------+---------+------+-----+---------+----------------+

#删除表
drop table table_name;

表记录操做

MariaDB [s18]> create table emp(
    ->     id int primary key auto_increment,
    ->     name varchar(20),
    ->     gender tinyint,
    ->     bir date,
    ->     salary float(6,2),
    ->     dep varchar(20));
Query OK, 0 rows affected (0.01 sec)

MariaDB [s18]> desc emp;
+--------+-------------+------+-----+---------+----------------+
| Field  | Type        | Null | Key | Default | Extra          |
+--------+-------------+------+-----+---------+----------------+
| id     | int(11)     | NO   | PRI | NULL    | auto_increment |
| name   | varchar(20) | YES  |     | NULL    |                |
| gender | tinyint(4)  | YES  |     | NULL    |                |
| bir    | date        | YES  |     | NULL    |                |
| salary | float(6,2)  | YES  |     | NULL    |                |
| dep    | varchar(20) | YES  |     | NULL    |                |
+--------+-------------+------+-----+---------+----------------+

1.增长表记录

MariaDB [s18]> insert emp (id,name,gender,bir,salary,dep) values
    ->                    (1,'tom',1,'1989-12-12',99.99,'安保部');

MariaDB [s18]> select * from emp;
+----+------+--------+------------+--------+-----------+
| id | name | gender | bir        | salary | dep       |
+----+------+--------+------------+--------+-----------+
|  1 | tom  |      1 | 1989-12-12 |  99.99 | 安保部    |
+----+------+--------+------------+--------+-----------+

MariaDB [s18]> insert emp (id,name,gender,bir,salary,dep) values 
    ->                    (2,'jerry',0,'1986-12-12',199.99,'安保部'),
    ->                    (3,'peter',1,'1988-12-12',2299.99,'高管部');
Query OK, 2 rows affected (0.00 sec)

MariaDB [s18]> select * from emp;
+----+-------+--------+------------+---------+-----------+
| id | name  | gender | bir        | salary  | dep       |
+----+-------+--------+------------+---------+-----------+
|  1 | tom   |      1 | 1989-12-12 |   99.99 | 安保部    |
|  2 | jerry |      0 | 1986-12-12 |  199.99 | 安保部    |
|  3 | peter |      1 | 1988-12-12 | 2299.99 | 高管部    |
+----+-------+--------+------------+---------+-----------+

#使用set也能够插入记录

MariaDB [s18]> insert emp set name='prog',gender=1,bir='1990-04-12',salary=8000,dep='教学部';

MariaDB [s18]> select * from emp;
+----+-------+--------+------------+---------+-----------+
| id | name  | gender | bir        | salary  | dep       |
+----+-------+--------+------------+---------+-----------+
|  1 | tom   |      1 | 1989-12-12 |   99.99 | 安保部    |
|  2 | jerry |      0 | 1986-12-12 |  199.99 | 安保部    |
|  3 | peter |      1 | 1988-12-12 | 2299.99 | 高管部    |
|  4 | prog  |      1 | 1990-04-12 | 8000.00 | 教学部    |
+----+-------+--------+------------+---------+-----------+

2.修改表记录

MariaDB [s18]> update emp set salary=300 where name='peter';

MariaDB [s18]> select * from emp;
+----+-------+--------+------------+---------+-----------+
| id | name  | gender | bir        | salary  | dep       |
+----+-------+--------+------------+---------+-----------+
|  1 | tom   |      1 | 1989-12-12 |   99.99 | 安保部    |
|  2 | jerry |      0 | 1986-12-12 |  199.99 | 安保部    |
|  3 | peter |      1 | 1988-12-12 |  300.00 | 高管部    |
|  4 | prog  |      1 | 1990-04-12 | 8000.00 | 教学部    |
+----+-------+--------+------------+---------+-----------+

3.删除表记录

MariaDB [s18]> delete from emp where id=2;

MariaDB [s18]> select * from emp;
+----+-------+--------+------------+---------+-----------+
| id | name  | gender | bir        | salary  | dep       |
+----+-------+--------+------------+---------+-----------+
|  1 | tom   |      1 | 1989-12-12 |   99.99 | 安保部    |
|  3 | peter |      1 | 1988-12-12 |  300.00 | 高管部    |
|  4 | prog  |      1 | 1990-04-12 | 8000.00 | 教学部    |
+----+-------+--------+------------+---------+-----------+

#清空表记录
1.delete from table_name
2.truncate table_name --> 适用于数据量很是大的表

MariaDB [s18]> truncate emp;

MariaDB [s18]> select * from emp;
Empty set (0.00 sec)

4.查询表记录

MariaDB [s18]> drop table emp;

MariaDB [s18]> create table emp(
    -> id      int primary key auto_increment,
    -> name    varchar(20),
    -> gender  enum('male','female','other'),
    -> age     tinyint,
    -> dep     varchar(20),
    -> city    varchar(20),
    -> salary  double(7,2));
Query OK, 0 rows affected (0.00 sec)

MariaDB [s18]> insert into emp (name,gender,age,dep,city,salary) values
    ->                         ('tom','male',24,'教学部','河北省',8000),
    ->                         ('jerry','male',34,'保安部','山东省',8000),
    ->                         ('peter','male',28,'保安部','山东省',10000),
    ->                         ('景丽阳','female',22,'教学部','北京',9000),
    ->                         ('张三','male',24,'教学部','河北省',6000),
    ->                         ('李四','male',32,'保安部','北京',12000),
    ->                         ('王五','male',38,'教学部','河北省',7000),
    ->                         ('赵六','male',28,'保安部','河北省',9000),
    ->                         ('猪七','female',24,'保洁部','北京',9000);
Query OK, 9 rows affected (0.01 sec)

MariaDB [s18]> select * from emp;
+----+-----------+--------+------+-----------+-----------+----------+
| id | name      | gender | age  | dep       | city      | salary   |
+----+-----------+--------+------+-----------+-----------+----------+
|  1 | tom       | male   |   24 | 教学部    | 河北省    |  8000.00 |
|  2 | jerry     | male   |   34 | 保安部    | 山东省    |  8000.00 |
|  3 | peter     | male   |   28 | 保安部    | 山东省    | 10000.00 |
|  4 | 景丽阳    | female |   22 | 教学部    | 北京      |  9000.00 |
|  5 | 张三      | male   |   24 | 教学部    | 河北省    |  6000.00 |
|  6 | 李四      | male   |   32 | 保安部    | 北京      | 12000.00 |
|  7 | 王五      | male   |   38 | 教学部    | 河北省    |  7000.00 |
|  8 | 赵六      | male   |   28 | 保安部    | 河北省    |  9000.00 |
|  9 | 猪七      | female |   24 | 保洁部    | 北京      |  9000.00 |
+----+-----------+--------+------+-----------+-----------+----------+

MariaDB [s18]> select name,salary from emp;
+-----------+----------+
| name      | salary   |
+-----------+----------+
| tom       |  8000.00 |
| jerry     |  8000.00 |
| peter     | 10000.00 |
| 景丽阳    |  9000.00 |
| 张三      |  6000.00 |
| 李四      | 12000.00 |
| 王五      |  7000.00 |
| 赵六      |  9000.00 |
| 猪七      |  9000.00 |
+-----------+----------+

MariaDB [s18]> select name as 姓名,salary as 薪水 from emp;  # as 能够省略  MariaDB [s18]> select name 姓名,salary 薪水 from emp;

+-----------+----------+
| 姓名      | 薪水     |
+-----------+----------+
| tom       |  8000.00 |
| jerry     |  8000.00 |
| peter     | 10000.00 |
| 景丽阳    |  9000.00 |
| 张三      |  6000.00 |
| 李四      | 12000.00 |
| 王五      |  7000.00 |
| 赵六      |  9000.00 |
| 猪七      |  9000.00 |
+-----------+----------+

MariaDB [s18]> select name from emp where age>20;
+-----------+
| name      |
+-----------+
| tom       |
| jerry     |
| peter     |
| 景丽阳    |
| 张三      |
| 李四      |
| 王五      |
| 赵六      |
| 猪七      |
+-----------+

MariaDB [s18]> select name,age from emp where age>20;
+-----------+------+
| name      | age  |
+-----------+------+
| tom       |   24 |
| jerry     |   34 |
| peter     |   28 |
| 景丽阳    |   22 |
| 张三      |   24 |
| 李四      |   32 |
| 王五      |   38 |
| 赵六      |   28 |
| 猪七      |   24 |
+-----------+------+

MariaDB [s18]> select name,age from emp where age between 20 and 30;
+-----------+------+
| name      | age  |
+-----------+------+
| tom       |   24 |
| peter     |   28 |
| 景丽阳    |   22 |
| 张三      |   24 |
| 赵六      |   28 |
| 猪七      |   24 |
+-----------+------+

MariaDB [s18]> select name,age from emp where name like '张%';
+--------+------+
| name   | age  |
+--------+------+
| 张三   |   24 |
+--------+------+

MariaDB [s18]> insert emp set name='张无忌';

MariaDB [s18]> select name,age from emp where name like '张%';
+-----------+------+
| name      | age  |
+-----------+------+
| 张三      |   24 |
| 张无忌    | NULL |
+-----------+------+

一个_表明一个符号

MariaDB [s18]> select name,age from emp where name like '张_';
+--------+------+
| name   | age  |
+--------+------+
| 张三   |   24 |
+--------+------+
1 row in set (0.00 sec)

MariaDB [s18]> select name,age from emp where name like '张__';
+-----------+------+
| name      | age  |
+-----------+------+
| 张无忌    | NULL |
+-----------+------+
1 row in set (0.00 sec)

MariaDB [s18]> select name,age from emp where gender='male' and age>25;
+--------+------+
| name   | age  |
+--------+------+
| jerry  |   34 |
| peter  |   28 |
| 李四   |   32 |
| 王五   |   38 |
| 赵六   |   28 |
+--------+------+

View Code

分组查询group by

MariaDB [s18]> delete from emp where name='张无忌';

MariaDB [s18]> select * from emp group by dep;
+----+--------+--------+------+-----------+-----------+---------+
| id | name   | gender | age  | dep       | city      | salary  |
+----+--------+--------+------+-----------+-----------+---------+
|  2 | jerry  | male   |   34 | 保安部    | 山东省    | 8000.00 |
|  9 | 猪七   | female |   24 | 保洁部    | 北京      | 9000.00 |
|  1 | tom    | male   |   24 | 教学部    | 河北省    | 8000.00 |
+----+--------+--------+------+-----------+-----------+---------+

此表是显示的每一个部门的第一我的的信息，没有用处

MariaDB [s18]> select dep from emp group by dep;
+-----------+
| dep       |
+-----------+
| 保安部    |
| 保洁部    |
| 教学部    |
+-----------+

MariaDB [s18]> select count(name) from emp group by dep;
+-------------+
| count(name) |
+-------------+
|           4 |
|           1 |
|           4 |
+-------------+
MariaDB [s18]> select count(*) from emp group by dep;    # 均可以
使用聚合函数查看每一个部门的总的人数

MariaDB [s18]> select dep,count(*) from emp group by dep;
+-----------+----------+
| dep       | count(*) |
+-----------+----------+
| 保安部    |        4 |
| 保洁部    |        1 |
| 教学部    |        4 |
+-----------+----------+

每一个部门具体的人数

MariaDB [s18]> select dep,max(salary) from emp group by dep;
+-----------+-------------+
| dep       | max(salary) |
+-----------+-------------+
| 保安部    |    12000.00 |
| 保洁部    |     9000.00 |
| 教学部    |     9000.00 |
+-----------+-------------+

查看每一个部门的最高工资

MariaDB [s18]> select gender,count(*) from emp group by gender;
+--------+----------+
| gender | count(*) |
+--------+----------+
| male   |        7 |
| female |        2 |
+--------+----------+

统计男生和女生的人数

MariaDB [s18]> select city,avg(salary) from emp group by city;
+-----------+--------------+
| city      | avg(salary)  |
+-----------+--------------+
| 北京      | 10000.000000 |
| 山东省    |  9000.000000 |
| 河北省    |  7500.000000 |
+-----------+--------------+

统计每一个省的员工的平均工资

MariaDB [s18]> select city,avg(salary) from emp group by city having avg(salary) > 8000;
+-----------+--------------+
| city      | avg(salary)  |
+-----------+--------------+
| 北京      | 10000.000000 |
| 山东省    |  9000.000000 |
+-----------+--------------+
查询平均工资大于8000的省份

View Code

查询条件顺序
where
group by
having ---> 对平均分组后的内容进行再过滤
order by
limit

MariaDB [s18]> select count(*) from emp;
+----------+
| count(*) |
+----------+
|        9 |
+----------+

查询公司全部员工的人数

MariaDB [s18]> select avg(salary) from emp;
+-------------+
| avg(salary) |
+-------------+
| 8666.666667 |
+-------------+

求整个公司的平均薪水

MariaDB [s18]> select * from emp order by salary;
+----+-----------+--------+------+-----------+-----------+----------+
| id | name      | gender | age  | dep       | city      | salary   |
+----+-----------+--------+------+-----------+-----------+----------+
|  5 | 张三      | male   |   24 | 教学部    | 河北省    |  6000.00 |
|  7 | 王五      | male   |   38 | 教学部    | 河北省    |  7000.00 |
|  1 | tom       | male   |   24 | 教学部    | 河北省    |  8000.00 |
|  2 | jerry     | male   |   34 | 保安部    | 山东省    |  8000.00 |
|  4 | 景丽阳    | female |   22 | 教学部    | 北京      |  9000.00 |
|  8 | 赵六      | male   |   28 | 保安部    | 河北省    |  9000.00 |
|  9 | 猪七      | female |   24 | 保洁部    | 北京      |  9000.00 |
|  3 | peter     | male   |   28 | 保安部    | 山东省    | 10000.00 |
|  6 | 李四      | male   |   32 | 保安部    | 北京      | 12000.00 |
+----+-----------+--------+------+-----------+-----------+----------+

MariaDB [s18]> select * from emp limit 2,4;
+----+-----------+--------+------+-----------+-----------+----------+
| id | name      | gender | age  | dep       | city      | salary   |
+----+-----------+--------+------+-----------+-----------+----------+
|  3 | peter     | male   |   28 | 保安部    | 山东省    | 10000.00 |
|  4 | 景丽阳    | female |   22 | 教学部    | 北京      |  9000.00 |
|  5 | 张三      | male   |   24 | 教学部    | 河北省    |  6000.00 |
|  6 | 李四      | male   |   32 | 保安部    | 北京      | 12000.00 |
+----+-----------+--------+------+-----------+-----------+----------+

从第2条下面开始取，往下取4条

MariaDB [s18]> select dep,group_concat(name) from emp group by dep;
+-----------+-----------------------------+
| dep       | group_concat(name)          |
+-----------+-----------------------------+
| 保安部    | jerry,peter,李四,赵六       |
| 保洁部    | 猪七                        |
| 教学部    | tom,景丽阳,张三,王五        |
+-----------+-----------------------------+

查看每一个组的员工姓名

View Code

多表查询

MariaDB [s18]> create table emp(
    ->     id int primary key auto_increment,
    ->     name varchar(20),
    ->     salary double(7,2),
    ->     dep_id int);
Query OK, 0 rows affected (0.00 sec)

MariaDB [s18]> insert emp (name,salary,dep_id) values ('张三',8000,2),
    ->                                                ('李四',12000,1),
    ->                                                ('王五',5000,2),
    ->                                                ('赵六',8000,3),
    ->                                                ('猪七',9000,1),
    ->                                                ('周八',7000,4),
    ->                                                ('蔡九',7000,2);
Query OK, 7 rows affected (0.01 sec)


MariaDB [s18]> create table dep(
    ->     id int primary key auto_increment,
    ->     name varchar(20)
    -> );
Query OK, 0 rows affected (0.01 sec)


MariaDB [s18]> insert into dep (name) values ('教学部'),
    ->                                       ('销售部'),
    ->                                       ('人事部');
Query OK, 3 rows affected (0.01 sec)

MariaDB [s18]> select * from emp;
+----+--------+----------+--------+
| id | name   | salary   | dep_id |
+----+--------+----------+--------+
|  1 | 张三   |  8000.00 |      2 |
|  2 | 李四   | 12000.00 |      1 |
|  3 | 王五   |  5000.00 |      2 |
|  4 | 赵六   |  8000.00 |      3 |
|  5 | 猪七   |  9000.00 |      1 |
|  6 | 周八   |  7000.00 |      4 |
|  7 | 蔡九   |  7000.00 |      2 |
+----+--------+----------+--------+

MariaDB [s18]> select * from dep;
+----+-----------+
| id | name      |
+----+-----------+
|  1 | 教学部    |
|  2 | 销售部    |
|  3 | 人事部    |
+----+-----------+

MariaDB [s18]> select * from emp,dep where emp.dep_id=dep.id;
+----+--------+----------+--------+----+-----------+
| id | name   | salary   | dep_id | id | name      |
+----+--------+----------+--------+----+-----------+
|  1 | 张三   |  8000.00 |      2 |  2 | 销售部    |
|  2 | 李四   | 12000.00 |      1 |  1 | 教学部    |
|  3 | 王五   |  5000.00 |      2 |  2 | 销售部    |
|  4 | 赵六   |  8000.00 |      3 |  3 | 人事部    |
|  5 | 猪七   |  9000.00 |      1 |  1 | 教学部    |
|  7 | 蔡九   |  7000.00 |      2 |  2 | 销售部    |
+----+--------+----------+--------+----+-----------+

MariaDB [s18]> select dep.name,emp.name from emp,dep where emp.dep_id=dep.id and emp.name='张三';
+-----------+--------+
| name      | name   |
+-----------+--------+
| 销售部    | 张三   |
+-----------+--------+

查询张三所在部门的名称
（此种方法并非之后常常使用的方法,有专门的语法 inner join--内链接查询）

MariaDB [s18]> select * from emp inner join dep on emp.dep_id=dep.id;
+----+--------+----------+--------+----+-----------+
| id | name   | salary   | dep_id | id | name      |
+----+--------+----------+--------+----+-----------+
|  1 | 张三   |  8000.00 |      2 |  2 | 销售部    |
|  2 | 李四   | 12000.00 |      1 |  1 | 教学部    |
|  3 | 王五   |  5000.00 |      2 |  2 | 销售部    |
|  4 | 赵六   |  8000.00 |      3 |  3 | 人事部    |
|  5 | 猪七   |  9000.00 |      1 |  1 | 教学部    |
|  7 | 蔡九   |  7000.00 |      2 |  2 | 销售部    |
+----+--------+----------+--------+----+-----------+

MariaDB [s18]> select * from emp inner join dep on emp.dep_id=dep.id where emp.name='张三';
+----+--------+---------+--------+----+-----------+
| id | name   | salary  | dep_id | id | name      |
+----+--------+---------+--------+----+-----------+
|  1 | 张三   | 8000.00 |      2 |  2 | 销售部    |
+----+--------+---------+--------+----+-----------+

左外链接查询left join（以左边的表为准）

MariaDB [s18]> select * from emp left join dep on emp.dep_id=dep.id;
+----+--------+----------+--------+------+-----------+
| id | name   | salary   | dep_id | id   | name      |
+----+--------+----------+--------+------+-----------+
|  1 | 张三   |  8000.00 |      2 |    2 | 销售部    |
|  2 | 李四   | 12000.00 |      1 |    1 | 教学部    |
|  3 | 王五   |  5000.00 |      2 |    2 | 销售部    |
|  4 | 赵六   |  8000.00 |      3 |    3 | 人事部    |
|  5 | 猪七   |  9000.00 |      1 |    1 | 教学部    |
|  6 | 周八   |  7000.00 |      4 | NULL | NULL      |
|  7 | 蔡九   |  7000.00 |      2 |    2 | 销售部    |
+----+--------+----------+--------+------+-----------+
比起inner join,多了  |  6 | 周八   |  7000.00 |      4 | NULL | NULL      |

右外链接查询right join（以右边的表为准）

MariaDB [s18]> select * from emp right join dep on emp.dep_id=dep.id;
+------+--------+----------+--------+----+-----------+
| id   | name   | salary   | dep_id | id | name      |
+------+--------+----------+--------+----+-----------+
|    1 | 张三   |  8000.00 |      2 |  2 | 销售部    |
|    2 | 李四   | 12000.00 |      1 |  1 | 教学部    |
|    3 | 王五   |  5000.00 |      2 |  2 | 销售部    |
|    4 | 赵六   |  8000.00 |      3 |  3 | 人事部    |
|    5 | 猪七   |  9000.00 |      1 |  1 | 教学部    |
|    7 | 蔡九   |  7000.00 |      2 |  2 | 销售部    |
+------+--------+----------+--------+----+-----------+

完整性约束

1.惟一约束--unique
惟一约束能够有多个但索引列的值必须惟一，索引列的值容许有空值

MariaDB [s18]> create table t1 (name varchar(20) unique);

MariaDB [s18]> insert t1 values ('tom');
Query OK, 1 row affected (0.00 sec)

MariaDB [s18]> insert t1 values ('tom');
ERROR 1062 (23000): Duplicate entry 'tom' for key 'name'

建立惟一约束后就不能插入相同的内容了

MariaDB [s18]> alter table t1 drop index name;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0


MariaDB [s18]> insert t1 values ('tom');
Query OK, 1 row affected (0.01 sec)

MariaDB [s18]> insert t1 values ('tom');
Query OK, 1 row affected (0.01 sec)

删除惟一索引后就能够插入重复数据了

MariaDB [s18]> alter table t1 add age int;
Query OK, 3 rows affected (0.02 sec)               
Records: 3  Duplicates: 0  Warnings: 0

MariaDB [s18]> alter table t1 add constraint UK_t1_age unique (age);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

2.自增约束
MySQL每张表只能有1个自动增加字段，这个自动增加字段一般做为主键，也能够用做
非主键使用，可是将自动增加字段看成非主键使用时必须为其添加惟一索引，不然系统报错

MariaDB [s18]> create table t2 (
    ->         id int not null,
    ->         name varchar(20),
    ->         age int auto_increment);
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key

改成如下：

MariaDB [s18]> create table t2 (
    ->         id int not null,
    ->         name varchar(20),
    ->         age int unique auto_increment);
Query OK, 0 rows affected (0.01 sec)

3.主键约束
非空且惟一

a.一张表不必定有主键
b.一张表最多只能有一个主键
c.表中若是没有设置主键，默认设置not null的字段为主键，此外，表中若是有多个not null的字段为主键，则
按顺序将第一个设置为not null的字段设为主键
d.主键必定是非空且惟一，但非空且惟一的字段不必定是主键
e.主键类型不必定必须是整型

MariaDB [s18]> create table t3(id int not null unique,name varchar(20));
Query OK, 0 rows affected (0.01 sec)

MariaDB [s18]> desc t3;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   | PRI | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

添加主键
alter table table_name add primary key(字段名称,...)
删除主键
alter table table_name drop primary key;

4.外键约束

表与表的关系是两张表的关系

一对多
关联字段要设在在数据多的那张表
主表：没有关联字段
子表：有关联字段

多对多
只能经过第三张表来创建关系

一对一
关联字段能够设在任何一张表上
将关联字段设置为unique

复合主键
主键含有一个以上的字段
若是一列不能惟一区分一张表里的记录，能够用多个组合起来达到区分表记录的惟一性

MariaDB [s18]> create table sc (
    ->             studentid int,
    ->             courseid int,
    ->             score int,
    ->             primary key (studentid,courseid)
    -> );


修改
alter table table_name add primary key (字段1,字段2...);