--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> describe user;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)
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
--外键约束；
--唯一约束；
--非空约束；
--默认约束；