约束条件与数据类型的宽度一样,都是可选参数
作用:用于保证数据的完整性和一致性 主要分为:
PRIMARY KEY (PK) 标识该字段为该表的主键,可以唯一的标识记录
FOREIGN KEY (FK) 标识该字段为该表的外键
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
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
UNSIGNED 无符号
ZEROFILL 使用0填充
说明:
1. 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
2. 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
sex enum('male','female') not null default 'male'
age int unsigned NOT NULL default 20 必须为正值(无符号) 不允许为空 默认是20
3. 是否是key
主键 primary key
外键 foreign key
索引 (index,unique...)
二 not null与default
是否可空,null表示空,非字符串 not null - 不可空 null - 可空
默认值,创建列时可以指定默认值,当插入数据时如果未主动设置,则自动添加默认值 create table tb1( nid int not null defalut 2, num int not null )
==================not null====================
mysql> create table t1(id int); #id字段默认可以插入空
mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
mysql> insert into t1 values(); #可以插入空
mysql> create table t2(id int not null); #设置字段id不为空
mysql> desc t2;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | NO | | NULL | |
+-------+---------+------+-----+---------+-------+
mysql> insert into t2 values(); #不能插入空
ERROR 1364 (HY000): Field 'id' doesn't have a default value
==================default====================
#设置id字段有默认值后,则无论id字段是null还是not null,都可以插入空,插入空默认填入default指定的默认值
mysql> create table t3(id int default 1);
mysql> alter table t3 modify id int not null default 1;
==================综合练习====================
mysql> create table student(
-> name varchar(20) not null,
-> age int(3) unsigned not null default 18,
-> sex enum('male','female') default 'male',
-> hobby set('play','study','read','music') default 'play,music'
-> );
mysql> desc student;
+-------+------------------------------------+------+-----+------------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------------------------------+------+-----+------------+-------+
| name | varchar(20) | NO | | NULL | |
| age | int(3) unsigned | NO | | 18 | |
| sex | enum('male','female') | YES | | male | |
| hobby | set('play','study','read','music') | YES | | play,music | |
+-------+------------------------------------+------+-----+------------+-------+
mysql> insert into student(name) values('egon');
mysql> select * from student;
+------+-----+------+------------+
| name | age | sex | hobby |
+------+-----+------+------------+
| egon | 18 | male | play,music |
+------+-----+------+------------+
============设置唯一约束 UNIQUE===============
方法一:
create table department1(
id int,
name varchar(20) unique,
comment varchar(100)
);
方法二:
create table department2(
id int,
name varchar(20),
comment varchar(100),
constraint uk_name unique(name)
);
mysql> insert into department1 values(1,'IT','技术');
Query OK, 1 row affected (0.00 sec)
mysql> insert into department1 values(1,'IT','技术');
ERROR 1062 (23000): Duplicate entry 'IT' for key 'name'
mysql> create table t1(id int not null unique);
Query OK, 0 rows affected (0.02 sec)
mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)
primary key字段的值不为空且唯一
一个表中可以:
单列做主键 多列做主键(复合主键)
但一个表内只能有一个主键primary key
============单列做主键===============
#方法一:not null+unique
create table department1(
id int not null unique, #主键
name varchar(20) not null unique,
comment varchar(100)
);
mysql> desc department1;
+---------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | NO | UNI | NULL | |
| comment | varchar(100) | YES | | NULL | |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)
#方法二:在某一个字段后用primary key
create table department2(
id int primary key, #主键
name varchar(20),
comment varchar(100)
);
mysql> desc department2;
+---------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
| comment | varchar(100) | YES | | NULL | |
+---------+--------------+------+-----+---------+-------+
rows in set (0.00 sec)
#方法三:在所有字段后单独定义primary key
create table department3(
id int,
name varchar(20),
comment varchar(100),
constraint pk_name primary key(id); #创建主键并为其命名pk_name
mysql> desc department3;
+---------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
| comment | varchar(100) | YES | | NULL | |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)
==================多列做主键================
create table service(
ip varchar(15),
port char(5),
service_name varchar(10) not null,
primary key(ip,port)
);
mysql> desc service;
+--------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+-------+
| ip | varchar(15) | NO | PRI | NULL | |
| port | char(5) | NO | PRI | NULL | |
| service_name | varchar(10) | NO | | NULL | |
+--------------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql> insert into service values
-> ('172.16.45.10','3306','mysqld'),
-> ('172.16.45.11','3306','mariadb')
-> ;
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> insert into service values ('172.16.45.10','3306','nginx');
ERROR 1062 (23000): Duplicate entry '172.16.45.10-3306' for key 'PRIMARY'
约束字段为自动增长,被约束的字段必须同时被key约束
#不指定id,则自动增长
create table student(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') default 'male'
);
mysql> desc student;
+-------+-----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-----------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
| sex | enum('male','female') | YES | | male | |
+-------+-----------------------+------+-----+---------+----------------+
mysql> insert into student(name) values
-> ('egon'),
-> ('alex')
-> ;
mysql> select * from student;
+----+------+------+
| id | name | sex |
+----+------+------+
| 1 | egon | male |
| 2 | alex | male |
+----+------+------+
#也可以指定id
mysql> insert into student values(4,'asb','female');
Query OK, 1 row affected (0.00 sec)
mysql> insert into student values(7,'wsb','female');
Query OK, 1 row affected (0.00 sec)
mysql> select * from student;
+----+------+--------+
| id | name | sex |
+----+------+--------+
| 1 | egon | male |
| 2 | alex | male |
| 4 | asb | female |
| 7 | wsb | female |
+----+------+--------+
#对于自增的字段,在用delete删除后,再插入值,该字段仍按照删除前的位置继续增长
mysql> delete from student;
Query OK, 4 rows affected (0.00 sec)
mysql> select * from student;
Empty set (0.00 sec)
mysql> insert into student(name) values('ysb');
mysql> select * from student;
+----+------+------+
| id | name | sex |
+----+------+------+
| 8 | ysb | male |
+----+------+------+
#应该用truncate清空表,比起delete一条一条地删除记录,truncate是直接清空表,在删除大表时用它
mysql> truncate student;
Query OK, 0 rows affected (0.01 sec)
mysql> insert into student(name) values('egon');
Query OK, 1 row affected (0.01 sec)
mysql> select * from student;
+----+------+------+
| id | name | sex |
+----+------+------+
| 1 | egon | male |
+----+------+------+
1 row in set (0.00 sec)
偏移量offset
员工信息表有三个字段:工号 姓名 部门
公司有3个部门,但是有1个亿的员工,那意味着部门这个字段需要重复存储,部门名字越长,越浪费
解决方法:
我们完全可以定义一个部门表
然后让员工信息表关联该表,如何关联,即foreign key
#表类型必须是innodb存储引擎,且被关联的字段,即references指定的另外一个表的字段,必须是主键
create table department(
id int primary key,
name varchar(20) not null
)engine=innodb;
#dpt_id外键,关联父表(department主键id),同步更新,同步删除
create table employee(
id int primary key,
name varchar(20) not null,
dpt_id int,
constraint fk_name foreign key(dpt_id)
references department(id)
on delete cascade
on update cascade
)engine=innodb;
#先往父表department中插入记录
insert into department values
(1,'欧德博爱技术有限事业部'),
(2,'艾利克斯人力资源部'),
(3,'销售部');
#再往子表employee中插入记录
insert into employee values
(1,'egon',1),
(2,'alex1',2),
(3,'alex2',2),
(4,'alex3',2),
(5,'李坦克',3),
(6,'刘飞机',3),
(7,'张火箭',3),
(8,'林子弹',3),
(9,'加特林',3)
;
#删父表department,子表employee中对应的记录跟着删
mysql> delete from department where id=3;
mysql> select * from employee;
+----+-------+--------+
| id | name | dpt_id |
+----+-------+--------+
| 1 | egon | 1 |
| 2 | alex1 | 2 |
| 3 | alex2 | 2 |
| 4 | alex3 | 2 |
+----+-------+--------+
#更新父表department,子表employee中对应的记录跟着改
mysql> update department set id=22222 where id=2;
mysql> select * from employee;
+----+-------+--------+
| id | name | dpt_id |
+----+-------+--------+
| 1 | egon | 1 |
| 3 | alex2 | 22222 |
| 4 | alex3 | 22222 |
| 5 | alex1 | 22222 |
+----+-------+--------+
表1 foreign key 表2
则表1的多条记录对应表2的一条记录,即多对一
利用foreign key的原理我们可以制作两张表的多对多,一对一关系
多对多:
表1的多条记录可以对应表2的一条记录
表2的多条记录也可以对应表1的一条记录
一对一:
表1的一条记录唯一对应表2的一条记录,反之亦然
分析时,我们先从按照上面的基本原理去套,然后再翻译成真实的意义,就很好理解了
三张表:出版社,作者信息,书
一对多(或多对一):一个出版社可以出版多本书
关联方式:foreign key
=====================多对一=====================
create table press(
id int primary key auto_increment,
name varchar(20)
);
create table book(
id int primary key auto_increment,
name varchar(20),
press_id int not null,
foreign key(press_id) references press(id)
on delete cascade
on update cascade
);
insert into press(name) values
('北京工业地雷出版社'),
('人民音乐不好听出版社'),
('知识产权没有用出版社')
;
insert into book(name,press_id) values
('九阳神功',1),
('九阴真经',2),
('九阴白骨爪',2),
('独孤九剑',3),
('降龙十巴掌',2),
('葵花宝典',3)
;
多对多:一个作者可以写多本书,一本书也可以有多个作者,双向的一对多,即多对多
关联方式:foreign key+一张新的表
=====================多对多=====================
create table author(
id int primary key auto_increment,
name varchar(20)
);
#这张表就存放作者表与书表的关系,即查询二者的关系查这表就可以了
create table author2book(
id int not null unique auto_increment,
author_id int not null,
book_id int not null,
constraint fk_author foreign key(author_id) references author(id)
on delete cascade
on update cascade,
constraint fk_book foreign key(book_id) references book(id)
on delete cascade
on update cascade,
primary key(author_id,book_id)
);
#插入四个作者,id依次排开
insert into author(name) values('egon'),('alex'),('yuanhao'),('wpq');
#每个作者与自己的代表作如下
1 egon:
1 九阳神功
2 九阴真经
3 九阴白骨爪
4 独孤九剑
5 降龙十巴掌
6 葵花宝典
2 alex:
1 九阳神功
6 葵花宝典
3 yuanhao:
4 独孤九剑
5 降龙十巴掌
6 葵花宝典
4 wpq:
1 九阳神功
insert into author2book(author_id,book_id) values
(1,1),
(1,2),
(1,3),
(1,4),
(1,5),
(1,6),
(2,1),
(2,6),
(3,4),
(3,5),
(3,6),
(4,1)
;
#两张表:学生表和客户表
一对一:一个学生是一个客户,一个客户有可能变成一个学校,即一对一的关系
关联方式:foreign key+unique
#一定是student来foreign key表customer,这样就保证了:
#1 学生一定是一个客户,
#2 客户不一定是学生,但有可能成为一个学生
create table customer(
id int primary key auto_increment,
name varchar(20) not null
);
create table student(
id int primary key auto_increment,
name varchar(20) not null,
class_name varchar(20) not null default 'python自动化',
level int default 1,
customer_id int unique, #该字段一定要是唯一的
foreign key(customer_id) references customer(id) #外键的字段一定要保证unique
on delete cascade
on update cascade
);
#增加客户
insert into customer(name) values
('李飞机'),
('王大炮'),
('守榴弹'),
('吴坦克'),
('赢火箭'),
('战地雷')
;
#增加学生
insert into student(name,customer_id) values
('李飞机',1),
('王大炮',2)
;
练习:账号信息表,用户组,主机表,主机组
#用户表
create table user(
id int not null unique auto_increment,
username varchar(20) not null,
password varchar(50) not null,
primary key(username,password)
);
insert into user(username,password) values
('root','123'),
('egon','456'),
('alex','alex3714')
;
#用户组表
create table usergroup(
id int primary key auto_increment,
groupname varchar(20) not null unique
);
insert into usergroup(groupname) values
('IT'),
('Sale'),
('Finance'),
('boss')
;
#主机表
create table host(
id int primary key auto_increment,
ip char(15) not null unique default '127.0.0.1'
);
insert into host(ip) values
('172.16.45.2'),
('172.16.31.10'),
('172.16.45.3'),
('172.16.31.11'),
('172.10.45.3'),
('172.10.45.4'),
('172.10.45.5'),
('192.168.1.20'),
('192.168.1.21'),
('192.168.1.22'),
('192.168.2.23'),
('192.168.2.223'),
('192.168.2.24'),
('192.168.3.22'),
('192.168.3.23'),
('192.168.3.24')
;
#业务线表
create table business(
id int primary key auto_increment,
business varchar(20) not null unique
);
insert into business(business) values
('轻松贷'),
('随便花'),
('大富翁'),
('穷一生')
;
#建关系:user与usergroup
create table user2usergroup(
id int not null unique auto_increment,
user_id int not null,
group_id int not null,
primary key(user_id,group_id),
foreign key(user_id) references user(id),
foreign key(group_id) references usergroup(id)
);
insert into user2usergroup(user_id,group_id) values
(1,1),
(1,2),
(1,3),
(1,4),
(2,3),
(2,4),
(3,4)
;
#建关系:host与business
create table host2business(
id int not null unique auto_increment,
host_id int not null,
business_id int not null,
primary key(host_id,business_id),
foreign key(host_id) references host(id),
foreign key(business_id) references business(id)
);
insert into host2business(host_id,business_id) values
(1,1),
(1,2),
(1,3),
(2,2),
(2,3),
(3,4)
;
#建关系:user与host
create table user2host(
id int not null unique auto_increment,
user_id int not null,
host_id int not null,
primary key(user_id,host_id),
foreign key(user_id) references user(id),
foreign key(host_id) references host(id)
);
insert into user2host(user_id,host_id) values
(1,1),
(1,2),
(1,3),
(1,4),
(1,5),
(1,6),
(1,7),
(1,8),
(1,9),
(1,10),
(1,11),
(1,12),
(1,13),
(1,14),
(1,15),
(1,16),
(2,2),
(2,3),
(2,4),
(2,5),
(3,10),
(3,11),
(3,12)
;