mysql 约束条件 auto_increment 自动增长

约束字段为自动增长，被约束的字段必须同时被key约束

id自动增长，每插入一条记录，自动增长

创建一张表 t20 id 字段设置为 不为空 唯一 自动增长

mysql> create table t20(id int not null unique auto_increment,name varchar(16));
Query OK, 0 rows affected (0.01 sec)

mysql> desc t20;
+-------+-------------+------+-----+---------+----------------+
| Field | Type        | Null | Key | Default | Extra          |
+-------+-------------+------+-----+---------+----------------+
| id    | int(11)     | NO   | PRI | NULL    | auto_increment |
| name  | varchar(16) | YES  |     | NULL    |                |
+-------+-------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

只对name字段插入记录，我们看到id自动递增

mysql> insert into t20(name) values('mike'),('alex'),('ben');
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from t20;
+----+------+
| id | name |
+----+------+
|  1 | mike |
|  2 | alex |
|  3 | ben  |
+----+------+
3 rows in set (0.00 sec)

针对id,name 插入也可以的

#也可以指定id 从7开始

mysql> insert into t20(id,name) values(7,'apple');
Query OK, 1 row affected (0.00 sec)

mysql> select * from t20;
+----+-------+
| id | name  |
+----+-------+
|  1 | mike  |
|  2 | alex  |
|  3 | ben   |
|  7 | apple |
+----+-------+

再针对name插入记录 id是从上次插入的记录开始计算

mysql> insert into t20(name) values('mike1'),('alex2'),('jack1');
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from t20;
+----+-------+
| id | name  |
+----+-------+
|  1 | mike  |
|  2 | alex  |
|  3 | ben   |
|  7 | apple |
|  8 | mike1 |
|  9 | alex2 |
| 10 | jack1 |
+----+-------+
7 rows in set (0.00 sec)