mysql建表约束

--mysql建表约束

－－主键约束
它能够唯一确定一张表中的内容，也就是我们通过某个字段添加约束，就可以是的该字段唯一(不重复)且不为空。
create table  user(
    id int primary key,
    name varchar(20)
    );
    －－联合主键
    只要联合的主键加起来不重复即可
    create table user1(
    id int,
    name varchar(20),
    password varchar(20),
    primary key(id,name)
    );
    insert into user1 values(1,'张三','123');
    insert into user1 values(2,'李四','123');
    insert into user1 values(3,'张三','123');
－－自增约束
--auto_increment
create table user2(
 id int primary key auto_increment,
 name varchar(20)
);
insert into user2 (name)values('张三')；
insert into user2 (name)values('李四')；
mysql> insert into user2 (name) values('张三');
Query OK, 1 row affected (0.03 sec)

mysql> insert into user2 (name) values('李四');
Query OK, 1 row affected (0.03 sec)

mysql> select * from user2;
+----+--------+
| id | name   |
+----+--------+
|  1 | 张三   |
|  2 | 李四   |
+----+--------+
会对id进行一个自动排序
2 rows in set (0.00 sec)
－－如果说在创建表时忘记创建主键约束，如何添加主键约束
create table user3(
id int,
name varchar(20)
);
desc user3;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

－－修改表结构
alter table user3 add primary key(id);
desc user3;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   | PRI | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

－－删除表结构

alter table user3 drop primary key;
desc user3;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
可以会看到user3中没有了primary key
－－使用modify修改字段，添加约束

－－外建约束
--涉及到两个表：父表，字表

--班级
create table classes(
    id int primary key,
    name varchar(20)
);
--学生表
create table student(
    id int primary key,
    name varchar(20),
    class_id int,
    foreign key(class_id) references classes(id)
);
mysql> desc classes;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   | PRI | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> desc student;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id       | int(11)     | NO   | PRI | NULL    |       |
| name     | varchar(20) | YES  |     | NULL    |       |
| class_id | int(11)     | YES  | MUL | NULL    |       |
+----------+-------------+------+-----+---------+-------+
3 rows in set (0.01 sec)
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');

insert into student values(1,'张三',1);
insert into student values(2,'李四',2);
insert into student values(3,'王五',3);
insert into student values(4,'赵流',4);
insert into student values(5,'赵流',5);
mysql> insert into student values(5,'赵流',5);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test`.`student`, CONSTRAINT `student_ibfk_1` FOREIGN KEY (`class_id`) REFERENCES `classes` (`id`))
--主表没有的元素，副表不可以进行引用
--主表中记录被副表引用的元素不能被删除
delete from classes where id = 4;
mysql> delete from classes where id = 4;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`test`.`student`, CONSTRAINT `student_ibfk_1` FOREIGN KEY (`class_id`) REFERENCES `classes` (`id`))
mysql> select * from classes;
+----+--------+
| id | name   |
+----+--------+
|  1 | 一班   |
|  2 | 二班   |
|  3 | 三班   |
|  4 | 四班   |
+----+--------+
4 rows in set (0.00 sec)

mysql> select * from student;
+----+--------+----------+
| id | name   | class_id |
+----+--------+----------+
|  1 | 张三   |        1 |
|  2 | 李四   |        2 |
|  3 | 王五   |        3 |
|  4 | 赵流   |        4 |
+----+--------+----------+
4 rows in set (0.00 sec)

mysql>

－－唯一约束
--约束修饰的字段的值不能重复

create table user5(
    id int,
    name varchar(20),
    primary key(id,name),:w
    
    unique(id)
);
mysql> desc user4;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  | UNI | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
desc user5;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   | PRI | NULL    |       |
| name  | varchar(20) | NO   | PRI | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
insert into user5 values(1,'张三')；
insert into user5 values(2,'李四')；
insert into user5 values(3,'王五')；
mysql> select * from user5;
+----+--------+
| id | name   |/
+----+--------+
|  1 | 张三   |
|  2 | 李四   |
|  3 | 王五   |
+----+--------+
3 rows in set (0.00 sec)
--如何删除唯一约束
alter table user5 drop index name;

－－非空约束
--修饰的字段不能为空
create table user6(
    id int,
    name varchar(20) not null
);
desc user6;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | NO   |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

－－默认约束
--就是当我们插入字段时，如果没有传值，就会使用默认值
create table user7(
    id int,
    name varchar(20),
    age int Default 10
);
mysql> desc user7;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
| age   | int(11)     | YES  |     | 10      |       |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
insert into user7 (id,name)values(1,'张三');
mysql> select * from user7;
+------+--------+------+
| id   | name   | age  |
+------+--------+------+
|    1 | 张三   |   10 |
+------+--------+------+
1 row in set (0.00 sec)