一 介绍

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203. 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 |+------+-----+------+------------+

三 unique

============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'

没加not nullmysql> create table teacher(    -> id int unique,    -> name char(10)    -> );Query OK, 0 rows affected (0.05 sec)mysql> desc teacher;+-------+----------+------+-----+---------+-------+| Field | Type     | Null | Key | Default | Extra |+-------+----------+------+-----+---------+-------+| id    | int(11)  | YES  | UNI | NULL    |       |                   #显示可以为空| name  | char(10) | YES  |     | NULL    |       |+-------+----------+------+-----+---------+-------+2 rows in set (0.00 sec)mysql> create table teacher1(    -> id int not null unique,    -> name char(10)    -> );Query OK, 0 rows affected (0.01 sec)+-------+----------+------+-----+---------+-------+| Field | Type     | Null | Key | Default | Extra |+-------+----------+------+-----+---------+-------+| id    | int(11)  | NO   | PRI | NULL    |       || name  | char(10) | YES  |     | NULL    |       |+-------+----------+------+-----+---------+-------+2 rows in set (0.00 sec)mysql> insert into teacher1 values(null,'egon');ERROR 1048 (23000): Column 'id' cannot be nullmysql> insert into teacher1 values(1,'egon');Query OK, 1 row affected (0.03 sec) mysql> insert into teacher1 values(1,'alex');        #不能在插入相同的id号ERROR 1062 (23000): Duplicate entry '1' for key 'id'

create table service(id int primary key auto_increment,name varchar(20),host varchar(15) not null,port int not null,unique(host,port) #联合唯一);mysql> insert into service values    -> (1,'nginx','192.168.0.10',80),    -> (2,'haproxy','192.168.0.20',80),    -> (3,'mysql','192.168.0.30',3306)    -> ;Query OK, 3 rows affected (0.01 sec)Records: 3  Duplicates: 0  Warnings: 0mysql> insert into service(name,host,port) values('nginx','192.168.0.10',80);ERROR 1062 (23000): Duplicate entry '192.168.0.10-80' for key 'host'

#多列唯一#255.255.255.255create table services(id int primary key auto_increment,name char(10),host char(15),port int,constraint host_port unique(host,port));insert into services values('ftp','192.168.20.17',8080);insert into services values('httpd','192.168.20.17',8081);

四 primary key

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+uniquecreate 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 keycreate 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 keycreate 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_namemysql> 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

==================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: 0mysql> insert into service values ('172.16.45.10','3306','nginx');ERROR 1062 (23000): Duplicate entry '172.16.45.10-3306' for key 'PRIMARY'

五 auto_increment

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idmysql> 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)

#在创建完表后，修改自增字段的起始值mysql> create table student(    -> id int primary key auto_increment,    -> name varchar(20),    -> sex enum('male','female') default 'male'    -> );mysql> alter table student auto_increment=3;mysql> show create table student;.......ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8mysql> insert into student(name) values('egon');Query OK, 1 row affected (0.01 sec)mysql> select * from student;+----+------+------+| id | name | sex  |+----+------+------+|  3 | egon | male |+----+------+------+row in set (0.00 sec)mysql> show create table student;.......ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8#也可以创建表时指定auto_increment的初始值，注意初始值的设置为表选项，应该放到括号外create table student(id int primary key auto_increment,name varchar(20),sex enum('male','female') default 'male')auto_increment=3;#设置步长sqlserver：自增步长    基于表级别    create table t1（        id int。。。    ）engine=innodb,auto_increment=2 步长=2 default charset=utf8mysql自增的步长：    show session variables like 'auto_inc%';        #基于会话级别    set session auth_increment_increment=2 #修改会话级别的步长    #基于全局级别的    set global auth_increment_increment=2 #修改全局级别的步长（所有会话都生效）#！！！注意了注意了注意了！！！If the value of auto_increment_offset is greater than that of auto_increment_increment, the value of auto_increment_offset is ignored. 翻译：如果auto_increment_offset的值大于auto_increment_increment的值，则auto_increment_offset的值会被忽略 比如：设置auto_increment_offset=3，auto_increment_increment=2mysql> set global auto_increment_increment=5;Query OK, 0 rows affected (0.00 sec)mysql> set global auto_increment_offset=3;Query OK, 0 rows affected (0.00 sec)mysql> show variables like 'auto_incre%'; #需要退出重新登录+--------------------------+-------+| Variable_name            | Value |+--------------------------+-------+| auto_increment_increment | 1     || auto_increment_offset    | 1     |+--------------------------+-------+create table student(id int primary key auto_increment,name varchar(20),sex enum('male','female') default 'male');mysql> insert into student(name) values('egon1'),('egon2'),('egon3');mysql> select * from student;+----+-------+------+| id | name  | sex  |+----+-------+------+|  3 | egon1 | male ||  8 | egon2 | male || 13 | egon3 | male |+----+-------+------+

六 foreign key

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，必须保证唯一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 cascadeon 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 |+----+-------+--------+mysql> update dep set id=301 where id=1;         Query OK, 1 row affected (0.04 sec)Rows matched: 1  Changed: 1  Warnings: 0mysql> select * from dep;               #当更改   父表的id后子表也会跟着更改+-----+------+---------+| id  | name | comment |+-----+------+---------+|   2 | ssss | aaaa    || 301 | ss   | aa      |+-----+------+---------+2 rows in set (0.00 sec)mysql> select * from emp_info;     #可以看到之前id为1现在id为301了+----+------+--------+| id | name | dep_id |+----+------+--------+|  1 | egon |    301 ||  2 | alex |      2 ||  3 | alex |      2 ||  4 | alex |      2 |+----+------+--------+4 rows in set (0.00 sec)

 

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

#一对多或称为多对一三张表：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

mysql> create table press(    -> id int primary key auto_increment,    -> name varchar(20)    -> );Query OK, 0 rows affected (0.01 sec)mysql> 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    -> );Query OK, 0 rows affected (0.01 sec)mysql> mysql> insert into press(name) values    -> ('北京工业地雷出版社'),    -> ('长沙马栏山精神出版社'); insert into book(name,press_id) values ('葵花宝典',1), ('如来神掌',2); mysql> select * from book;+----+---------------+----------+| id | name          | press_id |+----+---------------+----------+|  1 | 葵花宝典      |        1 ||  2 | 如来神掌      |        2 ||  3 | 葵花宝典q     |        1 ||  4 | 如来神掌w     |        2 |+----+---------------+----------+4 rows in set (0.00 sec)

#多对多三张表：出版社，作者信息，书多对多：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 cascadeon update cascade,constraint fk_book foreign key(book_id) references book(id)on delete cascadeon 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);

#一对一两张表：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uniqueon delete cascadeon 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);