mysql --键表约束

--mysql 键表约束，sql

--主键约束；
它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，就可以使得该字段不重复，且不为空。
有且只有一个

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

mysql> insert into user values(1,'zhangsan');
Query OK, 1 row affected (0.00 sec)

mysql> insert into user values(1,'zhangsan');
ERROR 1062 (23000): Duplicate entry '1' for key 'user.PRIMARY'
mysql>

mysql> insert into user values(1,'zhangsan');
ERROR 1062 (23000): Duplicate entry '1' for key 'user.PRIMARY'
mysql> insert into user values(2,'zhangsan');
Query OK, 1 row affected (0.00 sec)

mysql> select * from user;
+----+----------+
| id | name |
+----+----------+
| 1 | zhangsan |
| 2 | zhangsan |
+----+----------+
2 rows in set (0.00 sec)

mysql>

mysql> insert into user values(null,'zhangsan');
ERROR 1048 (23000): Column 'id' cannot be null
mysql>

--联合主键；
--只要联合主键值加起来不重复就可以,也不可以为空；
mysql> create table user2(
-> id int,
-> name varchar(20),
-> password varchar(20),
-> primary key(id,name)
-> );
Query OK, 0 rows affected (0.01 sec)

mysql> select * from user2;
+----+----------+----------+
| id | name | password |
+----+----------+----------+
| 1 | lisi | 234 |
| 1 | zhangsan | 123 |
| 2 | zhangsan | 123 |
+----+----------+----------+
3 rows in set (0.00 sec)

--自增约束
mysql> create table user3(
-> id int primary key auto_increment,
-> name varchar(20)
-> );
Query OK, 0 rows affected (0.01 sec)

mysql> insert into user3 (name) values('zhangsan');
Query OK, 1 row affected (0.01 sec)

mysql> select * from user3;
+----+----------+
| id | name |
+----+----------+
| 1 | zhangsan |
+----+----------+
1 row in set (0.00 sec)
mysql> insert into user3 (name) values('zhangsan');
Query OK, 1 row affected (0.00 sec)

mysql> select * from user3;
+----+----------+
| id | name |
+----+----------+
| 1 | zhangsan |
| 2 | zhangsan |
+----+----------+
2 rows in set (0.00 sec)

--> 就是自我增长

----> 在上面的基础上添加主键约束：通过alter
mysql> alter table user4 add primary key(id);
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql>

--如何删除
alter table user4 drop primary key;
uery OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql>

使用modify 修改字段，添加约束；

mysql> alter table user4 modify id int primary key;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql>

--唯一约束
--约束修饰的字段不可以重复，

mysql> alter table user5 add unique(name);
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql>

mysql> insert into user5 values(1,"zhangsan");
Query OK, 1 row affected (0.00 sec)

mysql> insert into user5 values(1,"zhangsan");
ERROR 1062 (23000): Duplicate entry 'zhangsan' for key 'user5.name'
mysql> insert into user5 values(1,"lisi");
Query OK, 1 row affected (0.00 sec)

mysql>

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

create table user8(
id int,
name varchar(20),
unique (id,name)
);

mysql>

mysql> insert into user8 values(1,"zhangsan");
Query OK, 1 row affected (0.00 sec)

mysql> insert into user8 values(1,"zhangsan");
ERROR 1062 (23000): Duplicate entry '1-zhangsan' for key 'user8.id'
mysql> insert into user8 values(2,"zhangsan");
Query OK, 1 row affected (0.00 sec)

mysql> select * from user8;
+------+----------+
| id | name |
+------+----------+
| 1 | zhangsan |
| 2 | zhangsan |
+------+----------+
2 rows in set (0.00 sec)

mysql>

----如何删除唯一约束；
alter tabble user7 drop index name;

mysql> alter table user7 drop index name;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql>

---通过modify 添加唯一约束；
alter table user7 modify name varchar(20) unique;

mysql> alter table user7 modify name varchar(20) unique;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql>

---总结

--1 建表的时候添加约束；
--2 可以使用alter .....add.....
--3 alter .....modify....
--4 s删除。alter 。。。。。drop...

--外键约束；

--唯一约束；

--非空约束；

--默认约束；