一、概述

为了约束用户对数据增，删，改，以确保数据正确，有效，合规。

有以下几种约束

not null 非空指定某列不能为空
unique 唯一指定某列或某几列的组合不能重复
primary key 主键指定某列的值可以唯一标识该列记录
foreign key 外键指定该行记录从属于主表的某条记录，主要用于参照完整性

二、not null

null 某列可以为空
not null 某列不可以为空

2.1 实例1

mysql> create table t2(id int not null) engine=innodb default charset = utf8;

Query OK, 0 rows affected (0.00 sec)

mysql> desc t2;

+-------+---------+------+-----+---------+-------+

| Field | Type    | Null | Key | Default | Extra |

+-------+---------+------+-----+---------+-------+

| id    | int(11) | NO   |     | NULL    |       |

+-------+---------+------+-----+---------+-------+

1 row in set (0.00 sec)

mysql> insert into t2(id) values(2);

Query OK, 1 row affected (0.01 sec)

mysql> select * from t2;

+----+

| id |

+----+

|  2 |

+----+

1 row in set (0.00 sec)

# 插入空值会报错

mysql> insert into t2(id) values(null);

ERROR 1048 (23000): Column 'id' cannot be null

2.2 与default 混用

mysql> create table t3(id1 int not null,id2 int not null default 10)engine = innodb default charset = utf8;

Query OK, 0 rows affected (0.00 sec)

mysql> desc t3;

+-------+---------+------+-----+---------+-------+

| Field | Type    | Null | Key | Default | Extra |

+-------+---------+------+-----+---------+-------+

| id1   | int(11) | NO   |     | NULL    |       |

| id2   | int(11) | NO   |     | 10      |       |

+-------+---------+------+-----+---------+-------+

2 rows in set (0.00 sec)


# 如果不指定默认列值，则使用默认值

mysql> insert into t3(id1) values(1);

Query OK, 1 row affected (0.01 sec)

mysql> select * from t3;

+-----+-----+

| id1 | id2 |

+-----+-----+

|   1 |  10 |

+-----+-----+

1 row in set (0.00 sec)


# 向默认值列插入数据，新数据会覆盖默认数据

mysql> insert into t3(id1,id2) values(12,12);

Query OK, 1 row affected (0.00 sec)

mysql> select * from t3;

+-----+-----+

| id1 | id2 |

+-----+-----+

|   1 |  10 |

|  12 |  12 |

+-----+-----+

三、unique

指定某列或某几列不能重复

3.1 创建唯一索引的两种方式

# 方法1

mysql> create table userinfo1(

    -> id int auto_increment primary key,

    -> name varchar(20) not null unique,

    -> password varchar(100) not null)

    -> engine = innodb default charset = utf8;

Query OK, 0 rows affected (0.01 sec)

mysql> desc userinfo1

    -> ;

+----------+--------------+------+-----+---------+----------------+

| Field    | Type         | Null | Key | Default | Extra          |

+----------+--------------+------+-----+---------+----------------+

| id       | int(11)      | NO   | PRI | NULL    | auto_increment |

| name     | varchar(20)  | NO   | UNI | NULL    |                |

| password | varchar(100) | NO   |     | NULL    |                |

方法2：

mysql> create table userinfo2(

    -> id int auto_increment primary key,

    -> name varchar(20),

    -> password varchar(100),

    -> unique(name))

    -> engine = innodb default charset = utf8;

Query OK, 0 rows affected (0.01 sec)

mysql> desc userinfo2;

+----------+--------------+------+-----+---------+----------------+

| Field    | Type         | Null | Key | Default | Extra          |

+----------+--------------+------+-----+---------+----------------+

| id       | int(11)      | NO   | PRI | NULL    | auto_increment |

| name     | varchar(20)  | YES  | UNI | NULL    |                |

| password | varchar(100) | YES  |     | NULL    |                |

+----------+--------------+------+-----+---------+----------------+

3 rows in set (0.00 sec)

# 向唯一索引所在的列进行插入数据时，如果数据相同，则会报错

mysql> insert into userinfo2(name,password) values('wangys','123');
Query OK, 1 row affected (0.00 sec)

mysql> insert into userinfo2(name,password) values('wangys','123');
ERROR 1062 (23000): Duplicate entry 'wangys' for key 'name'

3.2 联合唯一索引

多列的组合唯一

mysql> create table service(

    -> id int auto_increment primary key ,

    -> name varchar(20) not null,

    -> host varchar(32) not null,

    -> port int not null,

    -> unique(host,port))

    -> engine = innodb default charset = utf8;

Query OK, 0 rows affected (0.01 sec)

mysql> desc service;

+-------+-------------+------+-----+---------+----------------+

| Field | Type        | Null | Key | Default | Extra          |

+-------+-------------+------+-----+---------+----------------+

| id    | int(11)     | NO   | PRI | NULL    | auto_increment |

| name  | varchar(20) | NO   |     | NULL    |                |

| host  | varchar(32) | NO   | MUL | NULL    |                |

| port  | int(11)     | NO   |     | NULL    |                |

+-------+-------------+------+-----+---------+----------------+

4 rows in set (0.00 sec)

mysql> insert into service(name,host,port) values('nginx','1.1.1.1',80);

Query OK, 1 row affected (0.00 sec)

mysql> insert into service(name,host,port) values('nginx','1.1.1.1',80);

ERROR 1062 (23000): Duplicate entry '1.1.1.1-80' for key 'host'

mysql> insert into service(name,host,port) values('nginx','1.1.1.1',90);

Query OK, 1 row affected (0.00 sec)

四、primary key

唯一值
非空
可以为一列，也可以为多列（联合主键）
可以再创建表时创建，也可以基于已创建的表进行增加

4.1 创建主键的方法

第一种

mysql> create table userinfo(

    -> id int auto_increment primary key,

    -> name varchar(20))

    -> engine = innodb default charset = utf8;

Query OK, 0 rows affected (0.09 sec)

mysql> desc userinfo

    -> ;

+-------+-------------+------+-----+---------+----------------+

| Field | Type        | Null | Key | Default | Extra          |

+-------+-------------+------+-----+---------+----------------+

| id    | int(11)     | NO   | PRI | NULL    | auto_increment |

| name  | varchar(20) | YES  |     | NULL    |                |

+-------+-------------+------+-----+---------+----------------+

2 rows in set (0.00 sec)

第二种

mysql> create table userinfo2(

    -> id int auto_increment,

    -> name varchar(20),

    -> primary key(id))

    -> engine = innodb default charset = utf8;

Query OK, 0 rows affected (0.01 sec)

mysql> desc userinfo2;

+-------+-------------+------+-----+---------+----------------+

| Field | Type        | Null | Key | Default | Extra          |

+-------+-------------+------+-----+---------+----------------+

| id    | int(11)     | NO   | PRI | NULL    | auto_increment |

| name  | varchar(20) | YES  |     | NULL    |                |

+-------+-------------+------+-----+---------+----------------+

2 rows in set (0.01 sec)

第三种修改

mysql> create table userinfo3( id int, name varchar(20)) engine = innodb default charset = utf8;

Query OK, 0 rows affected (0.01 sec)

mysql> alter table userinfo3 modify id int auto_increment primary key ;

Query OK, 0 rows affected (0.02 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> desc userinfo3;

+-------+-------------+------+-----+---------+----------------+

| Field | Type        | Null | Key | Default | Extra          |

+-------+-------------+------+-----+---------+----------------+

| id    | int(11)     | NO   | PRI | NULL    | auto_increment |

| name  | varchar(20) | YES  |     | NULL    |                |

+-------+-------------+------+-----+---------+----------------+

2 rows in set (0.00 sec)

第四种联合主键

mysql> create table services(

    -> name varchar(20),

    -> host varchar(32),

    -> port int,

    -> primary key(host,port))

    -> engine = innodb default charset=utf8;

Query OK, 0 rows affected (0.00 sec)

mysql> desc services;

+-------+-------------+------+-----+---------+-------+

| Field | Type        | Null | Key | Default | Extra |

+-------+-------------+------+-----+---------+-------+

| name  | varchar(20) | YES  |     | NULL    |       |

| host  | varchar(32) | NO   | PRI |         |       |

| port  | int(11)     | NO   | PRI | 0       |       |

+-------+-------------+------+-----+---------+-------+

3 rows in set (0.00 sec)

mysql> insert into services(name,host,port) values('nginx','1.1.1.1',80);

Query OK, 1 row affected (0.00 sec)

mysql> insert into services(name,host,port) values('nginx','1.1.1.1',90);

Query OK, 1 row affected (0.00 sec)

mysql> insert into services(name,host,port) values('nginx','1.1.1.1',90);

ERROR 1062 (23000): Duplicate entry '1.1.1.1-90' for key 'PRIMARY'

4.2 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

mysql> create table userinfo4(

    -> id int primary key auto_increment,

    -> name varchar(20))

    -> engine = innodb default charset = utf8;

Query OK, 0 rows affected (0.00 sec)

mysql> desc userinfo4;

+-------+-------------+------+-----+---------+----------------+

| Field | Type        | Null | Key | Default | Extra          |

+-------+-------------+------+-----+---------+----------------+

| id    | int(11)     | NO   | PRI | NULL    | auto_increment |

| name  | varchar(20) | YES  |     | NULL    |                |

+-------+-------------+------+-----+---------+----------------+

2 rows in set (0.00 sec)

mysql> insert into userinfo4(name) values('wangys');

Query OK, 1 row affected (0.00 sec)

mysql> select * from userinfo4;

+----+--------+

| id | name   |

+----+--------+

|  1 | wangys |

+----+--------+

1 row in set (0.00 sec)

# 也可以直接指定id值

mysql> insert into userinfo4(id,name) values(4,'wangys');

Query OK, 1 row affected (0.00 sec)

mysql> select * from userinfo4;

+----+--------+

| id | name   |

+----+--------+

|  1 | wangys |

|  4 | wangys |

+----+--------+

五、外键

被关联的字段必须有唯一约束，且为innodb表

# 创建一个部门表
mysql> create table departments(

    -> id int auto_increment primary key,

    -> name varchar(25) not null)

    -> engine = innodb default charset = utf8;

Query OK, 0 rows affected (0.00 sec)

mysql> desc departments;

+-------+-------------+------+-----+---------+----------------+

| Field | Type        | Null | Key | Default | Extra          |

+-------+-------------+------+-----+---------+----------------+

| id    | int(11)     | NO   | PRI | NULL    | auto_increment |

| name  | varchar(25) | NO   |     | NULL    |                |

+-------+-------------+------+-----+---------+----------------+

2 rows in set (0.00 sec)

# 创建一个员工表dpt_id是外键，主表是部门表，关联id列

mysql> create table employee(

    -> id int auto_increment primary key,

    -> name varchar(25) not null,

    -> dpt_id int,

    -> foreign key(dpt_id)

    -> references departments(id)

    -> on delete cascade

    -> on update cascade)

    -> engine = innodb default charset = utf8;

Query OK, 0 rows affected (0.00 sec)

mysql> desc employee;

+--------+-------------+------+-----+---------+----------------+

| Field  | Type        | Null | Key | Default | Extra          |

+--------+-------------+------+-----+---------+----------------+

| id     | int(11)     | NO   | PRI | NULL    | auto_increment |

| name   | varchar(25) | NO   |     | NULL    |                |

| dpt_id | int(11)     | YES  | MUL | NULL    |                |

+--------+-------------+------+-----+---------+----------------+

# 删除主表某一行，则自动删除员工表关联的数据

mysql> select * from departments;

+----+--------+

| id | name   |

+----+--------+

|  1 | 产品   |

|  2 | 技术   |

|  3 | 销售   |

+----+--------+

3 rows in set (0.00 sec)

mysql> select * from employee;

+----+--------+--------+

| id | name   | dpt_id |

+----+--------+--------+

|  1 | 老王   |      2 |

|  3 | 老搜   |      3 |

|  4 | 老郭   |      1 |

+----+--------+--------+

3 rows in set (0.00 sec)

mysql> delete from departments where name ='产品';

Query OK, 1 row affected (0.00 sec)

mysql> select * from departments;

+----+--------+

| id | name   |

+----+--------+

|  2 | 技术   |

|  3 | 销售   |

+----+--------+

2 rows in set (0.00 sec)

mysql> select * from employee;

+----+--------+--------+

| id | name   | dpt_id |

+----+--------+--------+

|  1 | 老王   |      2 |

|  3 | 老搜   |      3 |

+----+--------+--------+

2 rows in set (0.00 sec)

Mysql-表的完整性约束的更多相关文章

三.mysql表的完整性约束
mysql表的完整性约束什么是约束 not null 不能为空的 unique 唯一 = 不能重复 primary key 主键 = 不能为空且不能重复 foreign key ...
mysql表的完整性约束
概览为了防止不符合规范的数据进入数据库,在用户对数据进行插入.修改.删除等操作时,DBMS自动按照一定的约束条件对数据进行监测, 使不符合规范的数据不能进入数据库,以确保数据库中存储的数据正确.有效 ...
day--38 mysql表的完整性约束总结
表的简单查询参考:https://www.cnblogs.com/clschao/articles/9995531.html 一:单表的查询: 查询数据的本质:mysql会去本地的硬盘上面找到对应的文 ...
MySQL表完整性约束
=======MySQL表完整性约束====== 目录: 一.介绍二.not null 与 default 三.unique 四.primary key 五.auto_increment 六.for ...
Mysql表基本操作
一. 创建表的方法语法:create table 表名( 属性名数据类型完整约束条件, 属性名数据类型条完整约束件, ......... 属性名数据类型 ); (1)举例:1 create tabl ...
MySQL表设计基础
MySQL表设计关于blog数据库中建立所有表的sql语句<一.>sql语句中约束概念constraint concept1.1 实体完整性entity integrity(主键--唯一 ...
MySQL中的完整性约束
对于已经创建好的表,虽然字段的数据类型决定所能存储的数据类型,但是表中所存储的数据是否合法并没有检查. MySQL支持的完整性约束: NOT NULL 约束字段的值不能 ...
数据库 -- mysql表操作
一,存储引擎介绍存储引擎即表类型,mysql根据不同的表类型会有不同的处理机制详见:https://www.cnblogs.com/peng104/p/9751738.html 二,表介绍表相当 ...
day36 mysql 表的具体操作
一库操作 1.创建数据库 1.1 语法 CREATE DATABASE 数据库名 charset utf8; 1.2 数据库命名规则可以由字母.数字.下划线.@.#.＄区分大小写唯一性 ...
Mysql数据库（三）Mysql表结构管理
一.MySQL数据类型 1.数字类型 (1)整数数据类型包括TINYINT/BIT/BOOL/SMALLINT/MEDIUMINT/INT/BIGINT (2)浮点数据类型包括FLOAT/DOUBLE ...

随机推荐

[BZOJ 3227] [SDOI 2008] 红黑树(tree)
Description 红黑树是一类特殊的二叉搜索树,其中每个结点被染成红色或黑色.若将二叉搜索树结点中的空指针看作是指向一个空结点,则称这类空结点为二叉搜索树的前端结点.并规定所有前端结点的高度为- ...
Linux服务器初步配置流程
一.root登录首先使用root用户登录远程主机: ssh -p prot root@host 这时命令行会输出类似的信息: The authenticity of host '[23.105.21 ...
Nginx从入门到实践(二)
静态资源web服务静态资源类型 CDN CDN的基本原理是广泛采用各种缓存服务器,将这些缓存服务器分布到用户访问相对集中的地区或网络中,在用户访问网站时,利用全局负载技术将用户的访问指向距离最近的工 ...
git学习03 - 撤销修改&删除文件
撤销修改:git checkout -- filename :将工作区文件回到最近一次add 或者 commit的状态撤销修改分为三种情况: 1.未提交至暂存区使用git checkout -- ...
Alice and Bob HDU - 4111 （SG函数）
Alice and Bob are very smart guys and they like to play all kinds of games in their spare time. The ...
DDCTF-2019
Web 滴 Web 签到题 Web 大吉大利,今晚吃鸡 1)滴网址http://117.51.150.246/index.php?jpg=TmpZMlF6WXhOamN5UlRaQk56QTJOdz ...
Day053--MySQL
MySQL安装和基本管理https://www.cnblogs.com/majj/p/9160383.html 管理员模式运行cmd 打开终端,输入mysqld,打开服务端. 打开终端,输入mysql ...
POJ 3666 Making the Grade (动态规划)
Description A straight dirt road connects two fields on FJ's farm, but it changes elevation more tha ...
libiconv交叉编译提示arm-none-linux-gnueabi-gcc
title: libiconv交叉编译提示arm-none-linux-gnueabi-gcc date: 2019/3/6 17:45:48 toc: true --- libiconv交叉编译提示 ...
前端安全类——CSRF/XSS
CSRF 概念:跨站请求伪造全称:Cross-site request forgery 攻击原理:网站中某一个接口存在漏洞,用户在注册网站登录过防御措施: 1.Token验证:引诱链接只会自动携带 ...

Mysql-表的完整性约束