Linux学习之MySQL建表

MySQL查询1 MySQL查询2

表管理

#1. 建库
  #1）库名命名规则
	  仅可以使用数字、字母、下划线、不能纯数字，区分字母大小写，具有唯一性，不可使用MySQL命令或特殊字符

#创建数据表时可以查看一下默认的字符集，8.0后创建数据库和表时默认字符集utf8mb4,5.7之前使用的是latin1，数据库默认编码不设置的话会使用server的字符集，
mysql> show variables like '%char%';
+--------------------------+----------------------------+
| Variable_name            | Value                      |
+--------------------------+----------------------------+
| character_set_client     | utf8mb4                    |
| character_set_connection | utf8mb4                    |
| character_set_database   | utf8mb4                    |
| character_set_filesystem | binary                     |
| character_set_results    | utf8mb4                    |
| character_set_server     | utf8mb4                    |
| character_set_system     | utf8mb3                    |
| character_sets_dir       | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+
8 rows in set (0.00 sec)

//库名区分字母大小写
mysql> create database gamedb ;
Query OK, 1 row affected (0.14 sec)
mysql> create database GAMEDB ;
Query OK, 1 row affected (0.08 sec)
mysql> create database GAMEDB ;  
ERROR 1007 (HY000): Can't create database 'GAMEDB'; database exists //重名报错
//加if not exists 命令避免重名报错
mysql> create database  if not exists  gamedb ;  
Query OK, 1 row affected, 1 warning (0.03 sec) //正常
mysql> show  databases; //查看创建的库
+--------------------+
| Database           |
+--------------------+
| GAMEDB             |
| gamedb             |
| information_schema |
| mysql              |
| performance_schema |
| sys                |
| tarena             |
+--------------------+
7 rows in set (0.00 sec)
mysql> drop database gamedb;  //删除库
Query OK, 0 rows affected (0.11 sec)
mysql> drop database gamedb; // 删除没有的库报错
ERROR 1008 (HY000): Can’t drop database ‘gamedb’; database doesn’t exist
//加if exists 删除没有的库，也不报错
mysql> drop database if exists gamedb;
Query OK, 0 rows affected, 1 warning (0.00 sec)
# 查看建库语句
mysql> create database studb;
Query OK, 1 row affected (0.10 sec)

mysql> use studb;
Database changed
mysql> show create database studb;
+----------+-----------------------------------------------------------------------------------------------------------------------------------+
| Database | Create Database                                                                                                                   |
+----------+-----------------------------------------------------------------------------------------------------------------------------------+
| student  | CREATE DATABASE `studb` /*!40100 DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci */ /*!80016 DEFAULT ENCRYPTION='N' */ |
+----------+-----------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
#2. 建表
mysql> create table stuinfo(name char(10),grade char(9),sex char(4),age int);
Query OK, 0 rows affected (0.94 sec)
#查看建表语句，COLLATE表示排序规则
mysql> show create table stuinfo;
+---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                                                       |
+---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| stuinfo | CREATE TABLE `stuinfo` (
  `name` char(10) DEFAULT NULL,
  `grade` char(9) DEFAULT NULL,
  `sex` char(4) DEFAULT NULL,
  `age` int DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
# 查看表结构信息
mysql> desc stuinfo;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| name  | char(10) | YES  |     | NULL    |       |
| grade | char(9)  | YES  |     | NULL    |       |
| sex   | char(4)  | YES  |     | NULL    |       |
| age   | int      | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
4 rows in set (0.00 sec)
# 删除表
mysql> drop table stuinfo;
Query OK, 0 rows affected (0.37 sec)
# 删除库
mysql> drop database student;
Query OK, 0 rows affected (0.18 sec)
#3. 修改表
 # 修改表名
mysql> alter table studb.stuinfo rename studb.student;
Query OK, 0 rows affected (0.31 sec)
# 查看数据库中包含哪些表
mysql> show tables;
+-----------------+
| Tables_in_studb |
+-----------------+
| student         |
+-----------------+
1 row in set (0.00 sec)
# 删除age表头
mysql> alter table studb.student drop age;
Query OK, 0 rows affected (0.84 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc studb.student;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| name  | char(10) | YES  |     | NULL    |       |
| grade | char(9)  | YES  |     | NULL    |       |
| sex   | char(4)  | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
3 rows in set (0.00 sec)
# 添加表头，默认添加在末尾
mysql> alter table studb.student add mail char(30);
Query OK, 0 rows affected (0.30 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc studb.student;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| name  | char(10) | YES  |     | NULL    |       |
| grade | char(9)  | YES  |     | NULL    |       |
| sex   | char(4)  | YES  |     | NULL    |       |
| mail  | char(30) | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
4 rows in set (0.00 sec)
# first指定添加列到行首，after 添加至指定表头名的下方
mysql> alter table studb.student add number char(30) first ;
Query OK, 0 rows affected (0.79 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc studb.student;
+--------+----------+------+-----+---------+-------+
| Field  | Type     | Null | Key | Default | Extra |
+--------+----------+------+-----+---------+-------+
| number | char(30) | YES  |     | NULL    |       |
| name   | char(10) | YES  |     | NULL    |       |
| grade  | char(9)  | YES  |     | NULL    |       |
| sex    | char(4)  | YES  |     | NULL    |       |
| mail   | char(30) | YES  |     | NULL    |       |
+--------+----------+------+-----+---------+-------+
5 rows in set (0.00 sec)
mysql> alter table studb.student add num1 char(30) after sex ;
Query OK, 0 rows affected (1.21 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc studb.student;
+--------+----------+------+-----+---------+-------+
| Field  | Type     | Null | Key | Default | Extra |
+--------+----------+------+-----+---------+-------+
| number | char(30) | YES  |     | NULL    |       |
| name   | char(10) | YES  |     | NULL    |       |
| grade  | char(9)  | YES  |     | NULL    |       |
| sex    | char(4)  | YES  |     | NULL    |       |
| num1   | char(30) | YES  |     | NULL    |       |
| mail   | char(30) | YES  |     | NULL    |       |
+--------+----------+------+-----+---------+-------+
6 rows in set (0.00 sec)
# 修改表头数据类型
mysql> alter table studb.student modify mail char(50);
Query OK, 0 rows affected (1.91 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc studb.student;
+--------+----------+------+-----+---------+-------+
| Field  | Type     | Null | Key | Default | Extra |
+--------+----------+------+-----+---------+-------+
| number | char(30) | YES  |     | NULL    |       |
| name   | char(10) | YES  |     | NULL    |       |
| grade  | char(9)  | YES  |     | NULL    |       |
| sex    | char(4)  | YES  |     | NULL    |       |
| num1   | char(30) | YES  |     | NULL    |       |
| mail   | char(50) | YES  |     | NULL    |       |
+--------+----------+------+-----+---------+-------+
6 rows in set (0.00 sec)
# 修改表头名
mysql> alter table studb.student change mail 邮箱 char(50);
Query OK, 0 rows affected (0.19 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc studb.student;
+--------+----------+------+-----+---------+-------+
| Field  | Type     | Null | Key | Default | Extra |
+--------+----------+------+-----+---------+-------+
| number | char(30) | YES  |     | NULL    |       |
| name   | char(10) | YES  |     | NULL    |       |
| grade  | char(9)  | YES  |     | NULL    |       |
| sex    | char(4)  | YES  |     | NULL    |       |
| num1   | char(30) | YES  |     | NULL    |       |
| 邮箱   | char(50) | YES  |     | NULL    |       |
+--------+----------+------+-----+---------+-------+
6 rows in set (0.00 sec)
#删除多个表头
mysql> alter table studb.student drop num1,drop grade;
Query OK, 0 rows affected (0.86 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc studb.student;
+--------+----------+------+-----+---------+-------+
| Field  | Type     | Null | Key | Default | Extra |
+--------+----------+------+-----+---------+-------+
| number | char(30) | YES  |     | NULL    |       |
| name   | char(10) | YES  |     | NULL    |       |
| sex    | char(4)  | YES  |     | NULL    |       |
| 邮箱   | char(50) | YES  |     | NULL    |       |
+--------+----------+------+-----+---------+-------+
4 rows in set (0.00 sec)
# 通过modify修改表头位置
mysql> alter table studb.student modify sex char(4) after number;
Query OK, 0 rows affected (1.53 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc studb.student;
+--------+----------+------+-----+---------+-------+
| Field  | Type     | Null | Key | Default | Extra |
+--------+----------+------+-----+---------+-------+
| number | char(30) | YES  |     | NULL    |       |
| sex    | char(4)  | YES  |     | NULL    |       |
| name   | char(10) | YES  |     | NULL    |       |
| 邮箱   | char(50) | YES  |     | NULL    |       |
+--------+----------+------+-----+---------+-------+
4 rows in set (0.00 sec)
# 复制表并将表内数据复制过来
mysql> create table studb.departments select * from tarena.departments;
Query OK, 11 rows affected (0.73 sec)
Records: 11  Duplicates: 0  Warnings: 0

mysql> desc studb.departments;
+-----------+-------------+------+-----+---------+-------+
| Field     | Type        | Null | Key | Default | Extra |
+-----------+-------------+------+-----+---------+-------+
| dept_id   | int         | NO   |     | 0       |       |
| dept_name | varchar(10) | YES  |     | NULL    |       |
+-----------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> select count(*) from studb.departments;
+----------+
| count(*) |
+----------+
|       11 |
+----------+
1 row in set (0.00 sec)

mysql> select count(*) from tarena.departments;
+----------+
| count(*) |
+----------+
|       11 |
+----------+
1 row in set (0.00 sec)

数据类型

# 1. 字符类型
mysql> create table studb.t2(name char(3),address varchar(5));
Query OK, 0 rows affected (0.68 sec)

mysql> desc studb.t2;
+---------+------------+------+-----+---------+-------+
| Field   | Type       | Null | Key | Default | Extra |
+---------+------------+------+-----+---------+-------+
| name    | char(3)    | YES  |     | NULL    |       |
| address | varchar(5) | YES  |     | NULL    |       |
+---------+------------+------+-----+---------+-------+
2 rows in set (0.01 sec)
mysql> insert into studb.t2 values('a','a');
Query OK, 1 row affected (0.12 sec)
mysql> insert into studb.t2 values('abc','abc');
Query OK, 1 row affected (0.10 sec)
# name定义为char(3),只能存储3个[字符]，所以报错数据太长了
mysql> insert into studb.t2 values('abcd','abcd');
ERROR 1406 (22001): Data too long for column 'name' at row 1
#mysql8.0使用了utf8mb4字符集，默认支持中文
mysql> insert into studb.t2 values("张无忌","武当山");
Query OK, 1 row affected (0.10 sec)
mysql> select * from studb.t2;
+-----------+-----------+
| name      | address   |
+-----------+-----------+
| a         | a         |
| ab        | ab        |
| abc       | abc       |
| 张无忌    | 武当山    |
+-----------+-----------+
4 rows in set (0.00 sec)
# 2. 数值类型
mysql> create table studb.t1(name char(10),level tinyint unsigned,money double);
Query OK, 0 rows affected (0.59 sec)

mysql> desc studb.t1;
+-------+------------------+------+-----+---------+-------+
| Field | Type             | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+-------+
| name  | char(10)         | YES  |     | NULL    |       |
| level | tinyint unsigned | YES  |     | NULL    |       |
| money | double           | YES  |     | NULL    |       |
+-------+------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
# tinyint unsigned表示无符号短整型，占用1个字节，只能存储0-255的数字
mysql> insert into studb.t1 values("战士",301,1.292);
ERROR 1264 (22003): Out of range value for column 'level' at row 1
mysql> insert into studb.t1 values("战士",255,1.292);
Query OK, 1 row affected (0.10 sec)
#整型不能存储不数，直接对小数进行四舍五入
mysql> insert into studb.t1 values("英雄",1.222,1.292);
Query OK, 1 row affected (0.08 sec)

mysql> select * from studb.t1;
+--------+-------+-------+
| name   | level | money |
+--------+-------+-------+
| 法师   |    80 |    88 |
| 战士   |   255 | 1.292 |
| 英雄   |     1 | 1.292 |
+--------+-------+-------+
3 rows in set (0.00 sec)
# 3. 枚举类型
mysql> create table studb.t8( name char(10),sex enum("男","女","保密"),hobby set("美女","吃","睡"));
Query OK, 0 rows affected (0.60 sec)

mysql> desc studb.t8;
+-------+----------------------------+------+-----+---------+-------+
| Field | Type                       | Null | Key | Default | Extra |
+-------+----------------------------+------+-----+---------+-------+
| name  | char(10)                   | YES  |     | NULL    |       |
| sex   | enum('男','女','保密')     | YES  |     | NULL    |       |
| hobby | set('美女','吃','睡')      | YES  |     | NULL    |       |
+-------+----------------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
# enum类型的数据只能选择其中一项，set类型的数据可以选择其中多项
mysql> insert into studb.t8 values("小迪总","男人","美女,睡,金钱");
ERROR 1265 (01000): Data truncated for column 'sex' at row 1
mysql> insert into studb.t8 values("小迪总","男","美女,睡,金钱");
ERROR 1265 (01000): Data truncated for column 'hobby' at row 1
mysql> insert into studb.t8 values("小迪总","男","美女,睡,吃");
Query OK, 1 row affected (0.12 sec)
mysql> select * from studb.t8;
+-----------+------+----------------+
| name      | sex  | hobby          |
+-----------+------+----------------+
| 小迪总    | 男   | 美女,吃,睡     |
+-----------+------+----------------+
1 row in set (0.00 sec)
# 4. 日期时间
mysql> create table studb.t6(name char(10),birth date,year_birth year,homeparty datetime,partyaddress varchar(15),worktime time);
Query OK, 0 rows affected (1.22 sec)

mysql> desc studb.t6;
+--------------+-------------+------+-----+---------+-------+
| Field        | Type        | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+-------+
| name         | char(10)    | YES  |     | NULL    |       |
| birth        | date        | YES  |     | NULL    |       |
| year_birth   | year        | YES  |     | NULL    |       |
| homeparty    | datetime    | YES  |     | NULL    |       |
| partyaddress | varchar(15) | YES  |     | NULL    |       |
| worktime     | time        | YES  |     | NULL    |       |
+--------------+-------------+------+-----+---------+-------+
6 rows in set (0.00 sec)
mysql> insert into studb.t6 values("小崔",19900101,1990,20230909090000,"郑州",100000);
Query OK, 1 row affected (0.11 sec)

mysql> select * from studb.t6;
+--------+------------+------------+---------------------+--------------+----------+
| name   | birth      | year_birth | homeparty           | partyaddress | worktime |
+--------+------------+------------+---------------------+--------------+----------+
| 小崔   | 1990-01-01 |       1990 | 2023-09-09 09:00:00 | 郑州         | 10:00:00 |
+--------+------------+------------+---------------------+--------------+----------+
1 row in set (0.00 sec)

数据批量处理

# 1.修改检索目录为/myload
# 查看默认检索目录
mysql> show variables like 'secure_file_priv';
+------------------+-----------------------+
| Variable_name    | Value                 |
+------------------+-----------------------+
| secure_file_priv | /var/lib/mysql-files/ |
+------------------+-----------------------+
1 row in set (0.00 sec)
# 此变量是只读变量，只能修改mysql配置文件，在mysqld下面添加一行secure_file_priv=/myload,并创建/myload目录，修改目录属主和属组为mysql,否则重启mysql服务时会报错
[root@mysql50 ~]# cat /etc/my.cnf.d/mysql-server.cnf 
[mysqld]
...
secure_file_priv=/myload
[root@mysql50 ~]# mkdir /myload
[root@mysql50 ~]# chown mysql.mysql /myload
[root@mysql50 ~]# systemctl restart mysqld
# 2.将/etc/passwd文件导入db1库的user3表里，并添加行号字段
#创建库和表
mysql> create database db1;
Query OK, 1 row affected (0.14 sec)
mysql> create table db1.user3(name varchar(30),password char(1),uid int ,gid int,comment varchar(200),homedir varchar(50),shell varchar(30));
Query OK, 0 rows affected (0.80 sec)

mysql> desc db1.user3;
+----------+--------------+------+-----+---------+-------+
| Field    | Type         | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+-------+
| name     | varchar(30)  | YES  |     | NULL    |       |
| password | char(1)      | YES  |     | NULL    |       |
| uid      | int          | YES  |     | NULL    |       |
| gid      | int          | YES  |     | NULL    |       |
| comment  | varchar(200) | YES  |     | NULL    |       |
| homedir  | varchar(50)  | YES  |     | NULL    |       |
| shell    | varchar(30)  | YES  |     | NULL    |       |
+----------+--------------+------+-----+---------+-------+
7 rows in set (0.01 sec)
# 拷贝文件到检索目录 system 在MySQL里执行系统命令
mysql> system cp /etc/passwd /myload/
mysql> system ls /myload
passwd
# 导入数据
mysql> load data infile "/myload/passwd" into table db1.user3 fields terminated by ":" lines terminated by "\n";
Query OK, 25 rows affected (0.13 sec)
Records: 25  Deleted: 0  Skipped: 0  Warnings: 0

mysql> select * from db1.user3;
+------------------+----------+-------+-------+-----------------------------+------------------+----------------+
| name             | password | uid   | gid   | comment                     | homedir          | shell          |
+------------------+----------+-------+-------+-----------------------------+------------------+----------------+
| root             | x        |     0 |     0 | root                        | /root            | /bin/bash      |
| bin              | x        |     1 |     1 | bin                         | /bin             | /sbin/nologin  |
| daemon           | x        |     2 |     2 | daemon                      | /sbin            | /sbin/nologin  |
| adm              | x        |     3 |     4 | adm                         | /var/adm         | /sbin/nologin  |
| lp               | x        |     4 |     7 | lp                          | /var/spool/lpd   | /sbin/nologin  |
| sync             | x        |     5 |     0 | sync                        | /sbin            | /bin/sync      |
| shutdown         | x        |     6 |     0 | shutdown                    | /sbin            | /sbin/shutdown |
| halt             | x        |     7 |     0 | halt                        | /sbin            | /sbin/halt     |
| mail             | x        |     8 |    12 | mail                        | /var/spool/mail  | /sbin/nologin  |
| operator         | x        |    11 |     0 | operator                    | /root            | /sbin/nologin  |
| games            | x        |    12 |   100 | games                       | /usr/games       | /sbin/nologin  |
| ftp              | x        |    14 |    50 | FTP User                    | /var/ftp         | /sbin/nologin  |
| nobody           | x        | 65534 | 65534 | Kernel Overflow User        | /                | /sbin/nologin  |
| dbus             | x        |    81 |    81 | System message bus          | /                | /sbin/nologin  |
| systemd-coredump | x        |   999 |   997 | systemd Core Dumper         | /                | /sbin/nologin  |
| systemd-resolve  | x        |   193 |   193 | systemd Resolver            | /                | /sbin/nologin  |
| polkitd          | x        |   998 |   995 | User for polkitd            | /                | /sbin/nologin  |
| unbound          | x        |   997 |   994 | Unbound DNS resolver        | /etc/unbound     | /sbin/nologin  |
| tss              | x        |    59 |    59 | Account used for TPM access | /dev/null        | /sbin/nologin  |
| chrony           | x        |   996 |   993 |                             | /var/lib/chrony  | /sbin/nologin  |
| sshd             | x        |    74 |    74 | Privilege-separated SSH     | /var/empty/sshd  | /sbin/nologin  |
| tcpdump          | x        |    72 |    72 |                             | /                | /sbin/nologin  |
| mysql            | x        |    27 |    27 | MySQL Server                | /var/lib/mysql   | /sbin/nologin  |
| apache           | x        |    48 |    48 | Apache                      | /usr/share/httpd | /sbin/nologin  |
| nginx            | x        |   995 |   992 | Nginx web server            | /var/lib/nginx   | /sbin/nologin  |
+------------------+----------+-------+-------+-----------------------------+------------------+----------------+
25 rows in set (0.00 sec)
# 3.将db1库user3表中所有记录导出，存到/myload/user.txt中
mysql> select * from db1.user3 into outfile "/myload/user.txt";
Query OK, 25 rows affected (0.00 sec)

mysql> system ls /myload
passwd  user.txt
mysql> system cat /myload/user.txt
root    x       0       0       root    /root   /bin/bash
bin     x       1       1       bin     /bin    /sbin/nologin
daemon  x       2       2       daemon  /sbin   /sbin/nologin
adm     x       3       4       adm     /var/adm        /sbin/nologin
lp      x       4       7       lp      /var/spool/lpd  /sbin/nologin
sync    x       5       0       sync    /sbin   /bin/sync
shutdown        x       6       0       shutdown        /sbin   /sbin/shutdown
halt    x       7       0       halt    /sbin   /sbin/halt
mail    x       8       12      mail    /var/spool/mail /sbin/nologin
operator        x       11      0       operator        /root   /sbin/nologin
games   x       12      100     games   /usr/games      /sbin/nologin
ftp     x       14      50      FTP User        /var/ftp        /sbin/nologin
nobody  x       65534   65534   Kernel Overflow User    /       /sbin/nologin
dbus    x       81      81      System message bus      /       /sbin/nologin
systemd-coredump        x       999     997     systemd Core Dumper     /       /sbin/nologin
systemd-resolve x       193     193     systemd Resolver        /       /sbin/nologin
polkitd x       998     995     User for polkitd        /       /sbin/nologin
unbound x       997     994     Unbound DNS resolver    /etc/unbound    /sbin/nologin
tss     x       59      59      Account used for TPM access     /dev/null       /sbin/nologin
chrony  x       996     993             /var/lib/chrony /sbin/nologin
sshd    x       74      74      Privilege-separated SSH /var/empty/sshd /sbin/nologin
tcpdump x       72      72              /       /sbin/nologin
mysql   x       27      27      MySQL Server    /var/lib/mysql  /sbin/nologin
apache  x       48      48      Apache  /usr/share/httpd        /sbin/nologin
nginx   x       995     992     Nginx web server        /var/lib/nginx  /sbin/nologin

表头基本约束

约束是一种限制，设置在表头上，用来控制表头的赋值
- NOT NULL：非空，用于保证该字段的值不能为空
- DEFAULT：默认值，用于保证该字段有默认值
- UNIQUE：唯一索引，用于保证该字段的值具有唯一性，可为空
- PRIMARY KEY：主键，用于保证字段的值具有唯一性且非空
- FOREIGN KEY：外键，用于限制两个表的关系，用于保证该字段的值必须来自于主表的关联列的值，在从表中添加外键约束，用于引用主表中的某些的值

# 1.表头不允许赋空值练习
mysql> create table db1.t33(name char(10) not null,grade char(7) default "nsd",likes set("money","game","film","music") not null default "film,music");
Query OK, 0 rows affected (0.64 sec)

mysql> desc db1.t33;
+-------+------------------------------------+------+-----+------------+-------+
| Field | Type                               | Null | Key | Default    | Extra |
+-------+------------------------------------+------+-----+------------+-------+
| name  | char(10)                           | NO   |     | NULL       |       |
| grade | char(7)                            | YES  |     | nsd        |       |
| likes | set('money','game','film','music') | NO   |     | film,music |       |
+-------+------------------------------------+------+-----+------------+-------+
3 rows in set (0.00 sec)

# 验证默认值和不允许为null
mysql> insert into db1.t33 values(null,null,null);
ERROR 1048 (23000): Column 'name' cannot be null
# 表头likes赋null值，报错
mysql> insert into db1.t33 values("bob",null,null);
ERROR 1048 (23000): Column 'likes' cannot be null
# 符合约束条件不报错
mysql> insert into db1.t33 values("bob",null,"money,game,film");
Query OK, 1 row affected (0.09 sec)
mysql> insert into db1.t33(name) values("lucy");
Query OK, 1 row affected (0.12 sec)
mysql> insert into db1.t33 values("jim","nsd2018","game,film");
Query OK, 1 row affected (0.17 sec)
mysql> select * from db1.t33;
+------+---------+-----------------+
| name | grade   | likes           |
+------+---------+-----------------+
| bob  | NULL    | money,game,film |
| lucy | nsd     | film,music      |
| jim  | nsd2018 | game,film       |
+------+---------+-----------------+
3 rows in set (0.00 sec)
# 2.表头加唯一索引练习
mysql> create table db1.t44(name char(10),passport char(18) unique);
Query OK, 0 rows affected (1.14 sec)

mysql> desc db1.t44;
+----------+----------+------+-----+---------+-------+
| Field    | Type     | Null | Key | Default | Extra |
+----------+----------+------+-----+---------+-------+
| name     | char(10) | YES  |     | NULL    |       |
| passport | char(18) | YES  | UNI | NULL    |       |
+----------+----------+------+-----+---------+-------+
2 rows in set (0.01 sec)

mysql> insert into db1.t44 values("bob",null);
Query OK, 1 row affected (0.13 sec)

mysql> insert into db1.t44 values("tom","666888");
Query OK, 1 row affected (0.08 sec)
# 表头值不可以重复
mysql> insert into db1.t44 values("jim","666888");
ERROR 1062 (23000): Duplicate entry '666888' for key 't44.passport'
mysql> insert into db1.t44 values("jim","766888");
Query OK, 1 row affected (0.16 sec)

mysql> select * from db1.t44;
+------+----------+
| name | passport |
+------+----------+
| bob  | NULL     |
| tom  | 666888   |
| jim  | 766888   |
+------+----------+
3 rows in set (0.00 sec)