mysql建表约束

Posted 2020-12-11

mysql建表约束

• 主键约束

他能够唯一确定一张表中的一条记录，也就是我们通过给某个字段添加约束，就可以使的该字段不重复且不为空

create table user(
id int primary key,
name varchar(20)
);

输入 describe user;

+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | NO   | PRI | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

这里的key值下的PRI即为主键约束

我们可以插入一个记录

insert into user values(1,‘张三‘);

再插入一次相同的记录

insert into user values(1,‘张三‘);

报错-------，是因为key值为1已经固定，不能再次插入key为1的记录,也不能为空

输入select * from user;

+----+------+
| id | name |
+----+------+
|  1 | 张三 |
+----+------+

还有一种情况，成为联合主键

> create table user2(
    -> id int,
    -> name varchar(20),
    -> password varchar(20),
    -> primary key(id,name) //表示id 和name 两个有一个不重复就可以
    -> );

• 自增约束

自动添加主键约束

create table user3(
    -> id int primary key auto_increment,
    -> name varchar(20)
    -> );

添加记录：insert into user3 (name) values(‘张三‘);

mysql> select * from user3;
+----+------+
| id | name |
+----+------+
|  1 | 张三 |
+----+------+

再次添加记录：insert into user3 (name) values(‘张三‘);

mysql> select * from user3;
+----+------+
| id | name |
+----+------+
|  1 | 张三 |
|  2 | 张三 |
+----+------+

如果创建表时忘记添加主键约束该怎么添加主键？

create table user4(
    -> id int,
    -> name varchar(20)
    -> );

decs user4

mysql> desc user4
    -> ;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

我们可以 alter table user4 add primary key(id);或者输入alter table user4 modify id int primary key;

mysql> desc user4
    -> ;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | NO   | PRI | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

同理，可以添加主键也可以删除主键：

alter table user4 drop primary key;

• 唯一约束

约束修饰的字段的值不可以重复

create table user5(
    -> id int,
    -> name varchar(20)
    -> );

desc user5
    -> ;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

输入：alter table user5 add unique(name);

mysql> desc user5
    -> ;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | YES  |     | NULL    |       |
| name  | varchar(20) | YES  | UNI | NULL    |       |
+-------+-------------+------+-----+---------+-------+

添加记录：insert into user values(1,‘张三‘);

mysql> select * from user5;
+----+------+
| id | name |
+----+------+
|  1 | 张三 |
+----+------+

再次添加记录：insert into user values(1,‘张三‘);

报错，重复

把名字改成李四就可以（insert into user values(1,‘李四‘);）

也可以在创建的时候直接添加约束

create table user6(
id int,
name varchar(20)
unique(name)
);

输出为

mysql> desc user6;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | YES  |     | NULL    |       |
| name  | varchar(20) | YES  | UNI | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

此外，unique还可以改变位置

 create table user7(
    -> id int,
    -> name varchar(20),unique
    -> );
 mysql> desc user7
    -> ;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| name  | varchar(20) | YES  | UNI | NULL    |       |
+-------+-------------+------+-----+---------+-------+

mysql> create table user8(
    -> id int,
    -> name varchar(20),
    -> unique(id,name)//用法同联合主键，有一个不重复的就可以
    -> );
Query OK, 0 rows affected (0.03 sec)

mysql> desc user8;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | YES  | MUL | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

• 如何删除唯一约束？

alter table user7 drop index name;

mysql> desc user7;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
1 row in set (0.00 sec)

• 如何添加唯一约束？

alter table user7 modify name varchar(20) unique;

• 非空约束

修饰的字段不能为空 NULL

mysql> create table user9(
    -> id int,
    -> name varchar(20) not null
    -> );

输出：

mysql> desc user9
    -> ;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | YES  |     | NULL    |       |
| name  | varchar(20) | NO   |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

可以看到 name值的null显示不能为空，即添加记录时name的null值不能为空，即

输入 insert into user9 values(1);会出错

输入insert into user9 values(1，’张三‘)就不会报错

• 默认约束

当插入字段值的时候，没有传值，就会使用默认值

mysql> create table user10(
    -> id int,
    -> name varchar(20),
    -> age int default 10
    -> );
Query OK, 0 rows affected (0.04 sec)

mysql> desc user10;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
| age   | int         | YES  |     | 10      |       |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> insert into user10 (id,name) values(1,‘zhangsan‘);//不传年龄就默认为10
Query OK, 1 row affected (0.01 sec)

mysql> select * from user10;
+------+----------+------+
| id   | name     | age  |
+------+----------+------+
|    1 | zhangsan |   10 |
+------+----------+------+

• 外键约束

涉及两个表：父表（主表）、子表（副表）

例如，有一个班级表（主表）

create table classes(
    -> id int primary key,
    -> name varchar(20)
    -> );

还有一个学生表（副表）

 create table students(
    -> id int primary key,
    -> name varchar(20),
    -> class_id int,
    -> foreign key(class_id) references classes(id)//指明副表里的id来自哪个表
    -> );

分别desc

 desc classes;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | NO   | PRI | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

desc students;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id       | int         | NO   | PRI | NULL    |       |
| name     | varchar(20) | YES  |     | NULL    |       |
| class_id | int         | YES  | MUL | NULL    |       |
+----------+-------------+------+-----+---------+-------+

首先往主表里插入数据：

insert into classes values(1,‘一班‘);

insert into classes values(2,‘二班‘);

insert into classes values(3,‘三班‘);

insert into classes values(4,‘四班‘);

查看数据------------select * from classes;

 select * from classes;
+----+------+
| id | name |
+----+------+
|  1 | 一班 |
|  2 | 二班 |
|  3 | 三班 |
|  4 | 四班 |
+----+------+

之后往副表里插入数据：

insert into students values(1001,‘甲‘，1);

insert into students values(1002,‘乙‘，2);

insert into students values(1003,‘丙‘，3);

insert into students values(1004,‘丁‘，4);

没有报错，但是 若insert into students values(1005,‘小明‘，5);就会报错，因为

主表中没有的数据值，在副表中是不可以使用的。

主表中的信息被副表引用，是不可以被删除的

总结自 [code158编程俱乐部]http://www.code158.com mysql编程系列教程