主键约束
能够唯一确定一张表中的一条记录，也就是我们通过给某一字段添加约束，就可以使得该字段不重复且不为空
mysql> use test
Database changed
mysql> create table user(
-> id int primary key,
-> name varchar(20)
-> );
Query OK, 0 rows affected (0.14 sec)
mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| pet |
| user |
+----------------+
2 rows in set (0.00 sec)
mysql> describe user;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.05 sec)
mysql> insert into user values('1','张三');
Query OK, 1 row affected (0.14 sec)
mysql> insert into user values('1','张三');
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
mysql> insert into user values('2','张三');
Query OK, 1 row affected (0.01 sec)
mysql> select * from user;
+----+------+
| id | name |
+----+------+
| 1 | 张三 |
| 2 | 张三 |
+----+------+
2 rows in set (0.00 sec)
mysql> insert into user values(NULL,'张三');
ERROR 1048 (23000): Column 'id' cannot be null
联合主键
只要联合的主键都不重复即可
mysql> create table user2(
-> id int,
-> name varchar(20),
-> password varchar(20),
-> primary key(id,name)
-> );
Query OK, 0 rows affected (0.12 sec)
mysql> insert into user2 values(1,'张三','123');
Query OK, 1 row affected (0.02 sec)
mysql> insert into user2 values(1,'张三','123');
ERROR 1062 (23000): Duplicate entry '1-张三' for key 'PRIMARY'
mysql> insert into user2 values(2,'张三','123');
Query OK, 1 row affected (0.10 sec)
mysql> insert into user2 values(1,'李四','123');
Query OK, 1 row affected (0.07 sec)
mysql> select * from user2;
+----+------+----------+
| id | name | password |
+----+------+----------+
| 1 | 张三 | 123 |
| 1 | 李四 | 123 |
| 2 | 张三 | 123 |
+----+------+----------+
3 rows in set (0.00 sec)