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)

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