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
);

 

null:

MariaDB [db5]> create table t1(id int, name char(5));
Query OK, 0 rows affected (0.01 sec)

MariaDB [db5]> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | YES  |     | NULL    |       |
| name  | char(5) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

MariaDB [db5]> insert into t1 values();   # 可以插入
Query OK, 1 row affected (0.00 sec)

MariaDB [db5]> select * from t1;
+------+------+
| id   | name |
+------+------+
| NULL | NULL |   # 查看到插入值均为 NULL
+------+------+
1 row in set (0.00 sec)

MariaDB [db5]>

结论:

当字段允许为null时,插入时没有填值(insert into t1 values();),那么默认插入值为NULL

 

not null:

MariaDB [db5]> create table t2(id int not null, name char(5) not null);  # 2个字段都设置为不允许为NULL
Query OK, 0 rows affected (0.01 sec)

MariaDB [db5]> desc t2;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   |     | NULL    |       |
| name  | char(5) | NO   |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

MariaDB [db5]> insert into t2 values();  # 可以插入,没有报错
Query OK, 1 row affected, 2 warnings (0.00 sec)

MariaDB [db5]> select * from t2;  # 查看结果,插入值还是null吗,并不是
+----+------+
| id | name |
+----+------+
|  0 |      |   # int类型默认插入值为0,char类型默认插入值为空字符串''
+----+------+
1 row in set (0.00 sec)

MariaDB [db5]> insert into t2 values(null,null);   # 直接插入null,就直接报错了
ERROR 1048 (23000): Column 'id' cannot be null
MariaDB [db5]> 
MariaDB [db5]> select * from t2 where name='';  # 验证char类型插入的是空字符串
+----+------+
| id | name |
+----+------+
|  0 |      |
+----+------+
1 row in set (0.00 sec)

MariaDB [db5]>

结论:

当字段不允许为null时 NOT NULL, 插入时没有填值(insert into t1 values();),插入时不会报错,但是int类型的字段默认插入值为0,char类型的字段默认插入值为空字符串”,而不是插入NULL,因为设置了不允许为NULL.

证明插入的是空字符串可通过  select * from t2 where name=”; 进行验证。

特别说明:null != ”   NULL不等于空字符串

 

default:

MariaDB [db5]> create table t3(id int not null default 100, name char(5) not null default "中国");
Query OK, 0 rows affected (0.00 sec)

MariaDB [db5]> desc t3;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   |     | 100     |       |
| name  | char(5) | NO   |     | 中国    |       |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

MariaDB [db5]> insert into t3 values();   # 当插入值为空时,插入默认值 id 为100,name为 中国
Query OK, 1 row affected (0.00 sec)

MariaDB [db5]> insert into t3 values(2,"北京");   # 手动设置插入值 id为2,name为北京
Query OK, 1 row affected (0.00 sec)

MariaDB [db5]> select * from t3;
+-----+--------+
| id  | name   |
+-----+--------+
| 100 | 中国   |
|   2 | 北京   |
+-----+--------+
2 rows in set (0.00 sec)

MariaDB [db5]>

 

综合练习:

MariaDB [db5]> create table t4(
    -> 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'
    -> );
Query OK, 0 rows affected (0.00 sec)

MariaDB [db5]> desc t4;
+-------+------------------------------------+------+-----+------------+-------+
| 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 |       |
+-------+------------------------------------+------+-----+------------+-------+
4 rows in set (0.00 sec)

MariaDB [db5]> insert into t4(name) values('alex');
Query OK, 1 row affected (0.00 sec)

MariaDB [db5]> select * from t4;
+------+-----+------+------------+
| name | age | sex  | hobby      |
+------+-----+------+------------+
| alex |  18 | male | play,music |
+------+-----+------+------------+
1 row in set (0.00 sec)

MariaDB [db5]>

 

三、 unique

设置唯一约束 unique:

# 方法一:
MariaDB [db5]> create table dep1(
    -> id int,
    -> name varchar(20) unique,
    -> comment varchar(100)
    -> );
Query OK, 0 rows affected (0.01 sec)

# 方法二:
MariaDB [db5]> create table dep2(
    -> id int,
    -> name varchar(20),
    -> comment varchar(100),
    -> constraint uk_name unique(name)
    -> );
Query OK, 0 rows affected (0.00 sec)

MariaDB [db5]> 

插入测试:

MariaDB [db5]> insert into dep1 values(1,'IT','技术');   # 第一次插入成功
Query OK, 1 row affected (0.00 sec)

MariaDB [db5]> insert into dep1 values(2,'IT','技术');   # 第二次插入 name和第一次相同,不能插入
ERROR 1062 (23000): Duplicate entry 'IT' for key 'name'
MariaDB [db5]>

not null + unique:

MariaDB [db5]> create table t5(id int not null unique);
Query OK, 0 rows affected (0.01 sec)

MariaDB [db5]> desc t5;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   | PRI | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)

MariaDB [db5]> insert into t5 values();
Query OK, 1 row affected, 1 warning (0.01 sec)

MariaDB [db5]> insert into t5 values();
ERROR 1062 (23000): Duplicate entry '0' for key 'id'
MariaDB [db5]> 
MariaDB [db5]> select * from t5;
+----+
| id |
+----+
|  0 |
+----+
1 row in set (0.00 sec)

MariaDB [db5]>

联合唯一:

MariaDB [db5]> 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
    -> );
Query OK, 0 rows affected (0.00 sec)

MariaDB [db5]> 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 (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

MariaDB [db5]> insert into service(name,host,port) values('nginx','192.168.0.10',80);   # 再次插入,表里已经有了192.168.0.10 和 80的组合,不能再次插入
ERROR 1062 (23000): Duplicate entry '192.168.0.10-80' for key 'host'
MariaDB [db5]> 

 

四、 primary key

对于innodb存储引擎来说,一张表内必须有一个主键。如果你没有设置主键,mysql会将表中不为空且唯一的字段自动设为主键,如过还没有不为空且唯一的字段,那么mysql会自己设置一个隐藏的主键。

 

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

MariaDB [db5]> create table dep3(
    -> id int not null unique,
    -> name varchar(20) not null unique,
    -> comment varchar(100)
    -> );
Query OK, 0 rows affected (0.01 sec)

MariaDB [db5]> desc dep3;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |   # 主键
| name    | varchar(20)  | NO   | UNI | NULL    |       |   # 唯一
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

方法二:在某一个字段后用primary key

MariaDB [db5]> create table dep4(
    -> id int primary key,  # 主键
    -> name varchar(20),
    -> comment varchar(100)
    -> );
Query OK, 0 rows affected (0.00 sec)

MariaDB [db5]> desc dep4;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |
| name    | varchar(20)  | YES  |     | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

方法三:在所有字段后单独定义primary key

MariaDB [db5]> create table dep5(
    -> 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
    -> );
Query OK, 0 rows affected (0.01 sec)

MariaDB [db5]> desc dep5;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | 0       |       |
| name    | varchar(20)  | YES  |     | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

 

多列做主键:

MariaDB [db5]> create table service2(
    -> ip varchar(15),
    -> port char(5),
    -> service_name varchar(10) not null,
    -> primary key(ip,port)
    -> );
Query OK, 0 rows affected (0.00 sec)

MariaDB [db5]> desc service2;
+--------------+-------------+------+-----+---------+-------+
| Field        | Type        | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+-------+
| ip           | varchar(15) | NO   | PRI |         |       |
| port         | char(5)     | NO   | PRI |         |       |
| service_name | varchar(10) | NO   |     | NULL    |       |
+--------------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

MariaDB [db5]> insert into service2 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

MariaDB [db5]> insert into service2 values ('172.16.45.10','3306','nginx');
ERROR 1062 (23000): Duplicate entry '172.16.45.10-3306' for key 'PRIMARY'
MariaDB [db5]> 

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

MariaDB [db5]> create table student(
    -> id int primary key auto_increment,
    -> name varchar(20),
    -> sex enum('male','female') default 'male'
    -> );
Query OK, 0 rows affected (0.01 sec)

MariaDB [db5]> 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    |                |
+-------+-----------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

# 插入值时只插入name字段,id字段自增,sex字段默认为 male
MariaDB [db5]> insert into student(name) values
    -> ('alex'),
    -> ('egon')
    -> ;
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

# 查看结果如下
MariaDB [db5]> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  1 | alex | male |
|  2 | egon | male |
+----+------+------+
2 rows in set (0.00 sec)

# 也可自己手动指定id,name,sex
MariaDB [db5]> insert into student values(4,"simon",'female');
Query OK, 1 row affected (0.00 sec)

MariaDB [db5]> 
MariaDB [db5]> select * from student;                         
+----+-------+--------+
| id | name  | sex    |
+----+-------+--------+
|  1 | alex  | male   |
|  2 | egon  | male   |
|  4 | simon | female |
+----+-------+--------+
3 rows in set (0.00 sec)

MariaDB [db5]> 

# 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

# 查看最新的id为4
MariaDB [db5]> select * from student;                         
+----+-------+--------+
| id | name  | sex    |
+----+-------+--------+
|  1 | alex  | male   |
|  2 | egon  | male   |
|  4 | simon | female |
+----+-------+--------+
3 rows in set (0.00 sec)

# 删除所有数据
MariaDB [db5]> delete from student;
Query OK, 3 rows affected (0.00 sec)

# 已删除所有数据
MariaDB [db5]> select * from student;
Empty set (0.00 sec)

# 重新插入一条数据
MariaDB [db5]> insert into student(name) values('sb');
Query OK, 1 row affected (0.01 sec)

# 新数据id从5开始
MariaDB [db5]> select * from student;                 
+----+------+------+
| id | name | sex  |
+----+------+------+
|  5 | sb   | male |
+----+------+------+
1 row in set (0.00 sec)

# 应该用truncate清空表,会重置索引,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。
MariaDB [db5]> truncate student;
Query OK, 0 rows affected (0.01 sec)

# 再次插入一条命令
MariaDB [db5]> insert into student(name) values('maria');
Query OK, 1 row affected (0.00 sec)

# id重新从1开始了
MariaDB [db5]> select * from student;                         
+----+-------+------+
| id | name  | sex  |
+----+-------+------+
|  1 | maria | male |
+----+-------+------+
1 row in set (0.00 sec)

MariaDB [db5]> 

修改自增字段的起始值:

方法一:创建表后进行修改

MariaDB [db5]> create table student(
    -> id int primary key auto_increment,
    -> name varchar(20),
    -> sex enum('male','female') default 'male'
    -> );
Query OK, 0 rows affected (0.00 sec)

MariaDB [db5]> show create table student;
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                                                |
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| student | CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  `sex` enum('male','female') DEFAULT 'male',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

MariaDB [db5]> 
MariaDB [db5]> alter table student auto_increment=3;  # 修改起始值为3
Query OK, 0 rows affected (0.01 sec)               
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [db5]> show create table student;           
+---------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                                                                 |
+---------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| student | CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  `sex` enum('male','female') DEFAULT 'male',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8 |    # 看到已改为3
+---------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

MariaDB [db5]> insert into student(name) values('alex');
Query OK, 1 row affected (0.00 sec)

# 插入一条新纪录,看到起始值确实为3
MariaDB [db5]> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  3 | alex | male |
+----+------+------+
1 row in set (0.00 sec)

MariaDB [db5]> 

方法二:创建表是指定auto_increment的大小

MariaDB [db5]> create table student(
    -> id int primary key auto_increment,
    -> name varchar(20),
    -> sex enum('male','female') default 'male'
    -> ) auto_increment=3;  # 在括号外
Query OK, 0 rows affected (0.01 sec)

MariaDB [db5]> show create table student;    
+---------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                                                                 |
+---------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| student | CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  `sex` enum('male','female') DEFAULT 'male',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8 |
+---------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

MariaDB [db5]> 

设置步长:

查看当前的步长:

MariaDB [db5]> show session variables like 'auto_inc%';
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 1     |
| auto_increment_offset    | 1     |
+--------------------------+-------+
2 rows in set (0.01 sec)
修改:
# 基于会话级别
MariaDB [db5]> set session auto_increment_increment=2;  # 修改会话级别的步长,只对当前会话生效
Query OK, 0 rows affected (0.00 sec)

# 基于全局级别的
MariaDB [db5]> set global auto_increment_increment=2;  # 修改会话级别的步长,对所有会话生效      
Query OK, 0 rows affected (0.00 sec)

需要退出重新登录才能看到改变。

 

#!!!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=2

 

MariaDB [db5]> set global auto_increment_increment=5;
Query OK, 0 rows affected (0.00 sec)

MariaDB [db5]> set global auto_increment_offset=3;
Query OK, 0 rows affected (0.00 sec)

MariaDB [db5]> show variables like 'auto_incre%';    # 需要重新登录才能看到生效
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 5     |
| auto_increment_offset    | 3     |
+--------------------------+-------+
2 rows in set (0.00 sec)
MariaDB [db5]> insert into student(name) values('alex'),('egon'),('wxx');
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

# 可以看到没增加一条记录id自增步长都为5
MariaDB [db5]> select * from student;                                    
+----+------+------+
| id | name | sex  |
+----+------+------+
|  3 | alex | male |
|  8 | egon | male |
| 13 | wxx  | male |
+----+------+------+
3 rows in set (0.00 sec)

 

六、 foreign key

一 快速理解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 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 |
+----+-------+--------+

 示范

二 如何找出两张表之间的关系

分析步骤:
#1、先站在左表的角度去找
是否左表的多条记录可以对应右表的一条记录,如果是,则证明左表的一个字段foreign key 右表一个字段(通常是id)

#2、再站在右表的角度去找
是否右表的多条记录可以对应左表的一条记录,如果是,则证明右表的一个字段foreign key 左表一个字段(通常是id)

#3、总结:
#多对一:
如果只有步骤1成立,则是左表多对一右表
如果只有步骤2成立,则是右表多对一左表

#多对多
如果步骤1和2同时成立,则证明这两张表时一个双向的多对一,即多对多,需要定义一个这两张表的关系表来专门存放二者的关系

#一对一:
如果1和2都不成立,而是左表的一条记录唯一对应右表的一条记录,反之亦然。这种情况很简单,就是在左表foreign key右表的基础上,将左表的外键字段设置成unique即可

三 建立表之间的关系

#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
=====================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)
;
一夫多妻制

#妻子表的丈夫id外键到丈夫表的id
#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 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
egon: 
九阳神功
九阴真经
九阴白骨爪
独孤九剑
降龙十巴掌
葵花宝典
alex: 
九阳神功
葵花宝典
yuanhao:
独孤九剑
降龙十巴掌
葵花宝典
wpq:
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)
;
单张表:用户表+相亲关系表,相当于:用户表+相亲关系表+用户表
多张表:用户表+用户与主机关系表+主机表

中间那一张存放关系的表,对外关联的字段可以联合唯一
#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,
qq varchar(10) not null,
phone char(16) not null
);


create table student(
id int primary key auto_increment,
class_name varchar(20) not null,
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,qq,phone) values
('李飞机','31811231',13811341220),
('王大炮','123123123',15213146809),
('守榴弹','283818181',1867141331),
('吴坦克','283818181',1851143312),
('赢火箭','888818181',1861243314),
('战地雷','112312312',18811431230)
;


#增加学生
insert into student(class_name,customer_id) values
('脱产3班',3),
('周末19期',4),
('周末19期',5)
;
例一:一个用户只有一个博客

    用户表:
    id  name
   egon
   alex
   wupeiqi


    博客表   
           fk+unique
    id url name_id
 xxxx   1
 yyyy   3
 zzz    2



例二:一个管理员唯一对应一个用户
    用户表:
    id user  password
 egon    xxxx
 alex    yyyy

    管理员表:
       fk+unique
    id user_id password
  1      xxxxx
  2      yyyyy

其他例子

 

 

 

 

 

 

 

发表评论

电子邮件地址不会被公开。

44 − = 40