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MySQL数据库的约束使用实例

时间：2022-12-06来源：www.pcxitongcheng.com作者：电脑系统城

1. NULL约束
2. UNIQUE(唯一约束)
3. DEFAULT(默认值约束)
4. PRIMARY KEY(主键约束)
5. FOREIGN KEY(外键约束)

数据库的约束就是关系型数据库给我们提供的一种"校验数据"合法性的机制

1. NULL约束

创建表时,可以指定某列不为空

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16 create table student(
id int not null,
sn int,
name varchar(20),
qq_mail varchar(20)
);
mysql> desc student;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id | int(11) | NO | | NULL | |
| sn | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| qq_mail | varchar(20) | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

2. UNIQUE(唯一约束)

保证某列的每行必须有唯一的值

重新设置一下表的结构

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15 create table student1(
id int not null,
sn int unique,
name varchar(20),
qq_mail varchar(20)
);
mysql> desc student1;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id | int(11) | NO | | NULL | |
| sn | int(11) | YES | UNI | NULL | |
| name | varchar(20) | YES | | NULL | |
| qq_mail | varchar(20) | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+

表示sn这列的值时唯一的不重复的

3. DEFAULT(默认值约束)

规定没有给列赋值时的默认值

指定插入数据时，name列为空，默认值unkown

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16 create table student2(
id int not null,
sn int unique,
name varchar(20) default 'unkown',
qq_mail varchar(20)
);
mysql> desc student2;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id | int(11) | NO | | NULL | |
| sn | int(11) | YES | UNI | NULL | |
| name | varchar(20) | YES | | unkown | |
| qq_mail | varchar(20) | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

插入一个id但不插入name时:

name是默认值

4. PRIMARY KEY(主键约束)

NOT NULL 和 UNIQUE 的结合

确保某列有唯一标识,有助于快速找到表中的一个特定记录

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16 create table student3(
id int not null primary key,
sn int unique,
name varchar(20) default 'unkown',
qq_mail varchar(20)
);
mysql> desc student3;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| sn | int(11) | YES | UNI | NULL | |
| name | varchar(20) | YES | | unkown | |
| qq_mail | varchar(20) | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

主键是not null 和 unique 的结合,也可以不用 not null

id int primary key

对于整数类型的主键,例如id,常搭配自增长auto_increment来使用

当插入数据对应字段不给定值时,使用最大值加一

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6 mysql> create table student4(
id int primary key auto_increment,
sn int unique,
name varchar(20) default 'unkown',
qq_mail varchar(20)
);

接下来我们插入记录时,id为null

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19 mysql> insert into student4 values (null,1000,'zhangsan',null);
Query OK, 1 row affected (0.00 sec)
mysql> insert into student4 values (null,1001,'zhangsi',null);
Query OK, 1 row affected (0.00 sec)
//将id置为100
insert into student4 values(100,1002,'zhangwu',null);
//将id置为null
insert into student4 values(null,1003,'zhangliu',null);
结果
mysql> select *from student4;
+-----+------+----------+---------+
| id | sn | name | qq_mail |
+-----+------+----------+---------+
| 1 | 1000 | zhangsan | NULL |
| 2 | 1001 | zhangsi | NULL |
| 100 | 1002 | zhangwu | NULL |
| 101 | 1003 | zhangliu | NULL |
+-----+------+----------+---------+
4 rows in set (0.00 sec)

可以看出,使用自增长之后,插入数据给值时,用给定的值插入,插入数据不给值时,默认插入最近一次的最大值+1

5. FOREIGN KEY(外键约束)

保证一个表中的数据匹配另一个表中的值的参照完整性

创建一个班级表class,id为主键.

创建学生表student，一个学生对应一个班级，一个班级对应多个学生。使用id为主键， classes_id为外键，关联班级表id

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15 create table class(
id int primary key,
name varchar(20),
`desc` VARCHAR(100)
);
Query OK, 0 rows affected (0.03 sec)
mysql> desc class;
+-------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
| desc | varchar(100) | YES | | NULL | |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

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19 create table student5(
id int primary key auto_increment,
sn int unique,name varchar(20) default 'unknow',
qq_mail varchar(20),
class_id int,
foreign key(class_id) references class(id)
);
Query OK, 0 rows affected (0.03 sec)
mysql> desc student5;
+----------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| sn | int(11) | YES | UNI | NULL | |
| name | varchar(20) | YES | | unknow | |
| qq_mail | varchar(20) | YES | | NULL | |
| class_id | int(11) | YES | MUL | NULL | |
+----------+-------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

此时学生表就和班级表关联起来了

外键约束是两个表之间的相互约束

我们插入一个非法数据,现在class的id列还是空的,插任何数据都会不合法

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5 insert into student5 values(1,1001,'张三',null,10);
ERROR 1452 (23000): Cannot add or update a child row:
a foreign key constraint fails (`java_3`.`student5`,
CONSTRAINT `student5_ibfk_1` FOREIGN KEY (`class_id`)
REFERENCES `class` (`id`))

插入失败,因为class的id列没有10这个数据

说明学生表中的数据依赖于班级表的数据,班级表的数据对学生表的数据产生约束力,班级表为父表,学生表为子表

我们先给班级表和学生表插入数据

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18 mysql> insert into class values(1,'java001',null);
Query OK, 1 row affected (0.00 sec)
mysql> insert into student5 values(1,1001,'张三',null,1);
Query OK, 1 row affected (0.00 sec)
mysql> select*from class;
+----+---------+------+
| id | name | desc |
+----+---------+------+
| 1 | java001 | NULL |
+----+---------+------+
1 row in set (0.00 sec)
mysql> select*from student5;
+----+------+------+---------+----------+
| id | sn | name | qq_mail | class_id |
+----+------+------+---------+----------+
| 1 | 1001 | 张三 | NULL | 1 |
+----+------+------+---------+----------+
1 row in set (0.00 sec)

因为数据都是合法的,所以成功插入

下面举例说明子表也会反过来约束父表

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6 mysql> delete from class where id = 1;
ERROR 1451 (23000):
Cannot delete or update a parent row:
a foreign key constraint fails (`java_3`.`student5`,
CONSTRAINT `student5_ibfk_1` FOREIGN KEY (`class_id`)
REFERENCES `class` (`id`))