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MySQL中数据的约束

Jun 07, 2016 pm 03:25 PM
mysqlnumberdatadatabasedata sheetconstraintTalk about it

今天说说数据库中的数据表数据约束问题,约束大概分为主键约束,外键约束,默认约束,唯一约束,非空约束这几种,关于讲解配置,我后面慢慢说 1、主键约束 其实是用来唯一标示表中的一个列或多个列,但是一个表中必须唯一,配置方法 (1)、初始化列级主键约

今天说说数据库中的数据表数据约束问题,约束大概分为主键约束,外键约束,默认值约束,唯一约束,非空约束这几种,关于讲解配置,我后面慢慢说

1、主键约束

其实是用来唯一标示表中的一个列或多个列,但是一个表中必须唯一,配置方法

(1)、初始化列级主键约束

create table custominfo
(
id int primary key,
name varchar(12),
age int
);

(2)、初始化表级主键约束
create table custominfo
(
id int,
name varchar(12),
age int,
primary key(id)
);

(3)、修改主键约束
alter table customerinfo
add constraint yueshu primary key(id);

(4)、修改添加联合主键约束
alter table customerinfo
add constraint yueshu primary key(id,name);

(5)、删除主键约束
alter table customerinfo
drop primary key;

2、外键约束

比如某个表B关联于表A,也就是表A中所有项目表B中都有,就可以设置表A主键关联,表B外键关联


(1)初始化添加

create table typeinfo
(
typeid int primary key,
type varchar(20)
);
create table roominfo
(
roomid int primary key,
type varchar(20),
price int
constraint yueshu foreign key(roomid) references typeinfo(typeid)
);

(2)、修改添加
alter table roominfo
add constraint yueshu foreign key(roomid) references typeinfo(typeid);

(3)、删除外键约束
alter table roominfo
drop foreign key yueshu;

3、默认值约束

将某一数据初始化为固定值

(1)、初始化设置

create table roominfo
(
id int,
name varchar(20),
price int default 0
);

(2)、修改添加设置
alter table roominfo
alter price set default 0;

(3)、删除默认约束
alter table roominfo
alter price drop default;

4、非空约束

某项数据不得为空的约束

(1)、初始化设置

create table roominfo
(
id int not null,
name varchar(20)
);

(2)、修改设置
alter table roominfo
alter id int not null;

5、检查约束

就是固定行规定数据的范围

(1)、初始化列级设置

<pre name="code" class="sql"><pre name="code" class="sql" style="background-color: rgb(255, 255, 255); "><pre name="code" class="sql">create table roominfo
(
id int,
name varchar(20),
price int check(price>=0)
);




(2)、初始化表级设置
<pre name="code" class="sql">create table roominfo
(
id int,
name varchar(20),
price int,
check(price>=0)
);


(3)、修改设置
alter table roominfo
add constraint yueshu check(price>=0);

6、唯一约束

(1)、初始化列级唯一约束

create table roominfo
(
id int unique,
name varchar(20)
);

(2)、初始化表级唯一约束
create table roominfo
(
id int,
name varchar(20),
unique(id)
);

可以设置多个独立唯一约束

(3)、初始化创建共同唯一约束

create table roominfo
(
id int,
name varchar(20),
price int,
unique(id,name)
);

(4)、修改设置独立唯一约束
alter table roominfo
add constraint yueshu unique(id);

(5)、修改设置共同唯一约束
alter table roominfo
add constraint yueshu unqiue(id,name);

(6)、删除唯一约束
drop index yueshu on roominfo;


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