Mysql-表的完整性约束

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)

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)