RDS for MySQL对自增字段赋值有以下几种方法：

# 表结构
CREATE TABLE animals (
     id MEDIUMINT NOT NULL AUTO_INCREMENT,
     name CHAR(30) NOT NULL,
     PRIMARY KEY (id)
);

不对自增字段赋值，数据库会自动将自增值填入字段中。AUTO_INCREMENT为自增。

mysql> INSERT INTO animals (name) VALUES ('fish'),('cat'),('penguin'),('lax'),('whale'),('ostrich');
Query OK, 6 rows affected (0.01 sec)
Records: 6  Duplicates: 0  Warnings: 0 
mysql> select * from animals;
+----+---------+
| id | name    |
+----+---------+
|  1 | fish     |
|  2 | cat     |
|  3 | penguin |
|  4 | lax     |
|  5 | whale   |
|  6 | ostrich |
+----+---------+
6 rows in set (0.00 sec) 
mysql> show create table animals;
+---------+--------------------------------------------------+
| Table   | Create Table                          |
+---------+--------------------------------------------------+
| animals | CREATE TABLE `animals` (  `id` mediumint NOT NULL AUTO_INCREMENT,  `name` char(30) NOT NULL,  PRIMARY KEY (`id`)) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8 |
+---------+--------------------------------------------------+

对自增字段赋0或null值，数据库会自动将自增值填入字段中。AUTO_INCREMENT为自增。

mysql> INSERT INTO animals (id,name) VALUES(0,'groundhog');
Query OK, 1 row affected (0.00 sec) 
mysql> INSERT INTO animals (id,name) VALUES(NULL,'squirrel');
Query OK, 1 row affected (0.01 sec) 
mysql> select * from animals;
+----+-----------+
| id | name      |
+----+-----------+
|  1 | fish       |
|  2 | cat       |
|  3 | penguin   |
|  4 | lax       |
|  5 | whale     |
|  6 | ostrich   |
|  7 | groundhog |
|  8 | squirrel  |
+----+-----------+8 
rows in set (0.00 sec) 
mysql> show create table animals;
+---------+----------------------------------------------------------------+
| Table   | Create Table                                                                             |
+---------+----------------------------------------------------------------+
| animals | CREATE TABLE `animals` (  `id` mediumint NOT NULL AUTO_INCREMENT,  `name` char(30) NOT NULL,  PRIMARY KEY (`id`)) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8 |
+---------+------------------------------------------------------------+

直接使用大于AUTO_INCREMENT的值X，数据库会将X填入字段并修改AUTO_INCREMENT=X+1。

mysql> INSERT INTO animals (id,name) VALUES(100,'rabbit');
Query OK, 1 row affected (0.00 sec) 
mysql> select * from animals;
+-----+-----------+
| id  | name      |
+-----+-----------+
|   1 | fish       |
|   2 | cat       |
|   3 | penguin   |
|   4 | lax       |
|   5 | whale     |
|   6 | ostrich   |
|   7 | groundhog |
|   8 | squirrel  |
| 100 | rabbit    |
+-----+-----------+
9 rows in set (0.00 sec) 
mysql> show create table animals;
+---------+----------------------------------------------------------------------+
| Table   | Create Table                                                                                      |
+---------+----------------------------------------------------------------------+
| animals | CREATE TABLE `animals` (  `id` mediumint NOT NULL AUTO_INCREMENT,  `name` char(30) NOT NULL,  PRIMARY KEY (`id`)) ENGINE=InnoDB AUTO_INCREMENT=101 DEFAULT CHARSET=utf8 |
+---------+----------------------------------------------------------------------+

直接使用小于AUTO_INCREMENT但不冲突的值。数据可以插入，但AUTO_INCREMENT不变。

mysql> INSERT INTO animals (id,name) VALUES(50,'middle');
Query OK, 1 row affected (0.00 sec) 
mysql> select * from animals;
+-----+-----------+
| id  | name      |
+-----+-----------+
|   1 | fish       |
|   2 | cat       |
|   3 | penguin   |
|   4 | lax       |
|   5 | whale     |
|   6 | ostrich   |
|   7 | groundhog |
|   8 | squirrel  |
|  50 | middle    |
| 100 | rabbit    |
+-----+-----------+
10 rows in set (0.00 sec) 
mysql> show create table animals;
+---------+------------------------------------------------------------------+
| Table   | Create Table                                                                               |
+---------+------------------------------------------------------------------+
| animals | CREATE TABLE `animals` (  `id` mediumint NOT NULL AUTO_INCREMENT,  `name` char(30) NOT NULL,  PRIMARY KEY (`id`)) ENGINE=InnoDB AUTO_INCREMENT=101 DEFAULT CHARSET=utf8 |
+---------+------------------------------------------------------------------+

直接使用负值。数据可以插入，但AUTO_INCREMENT不变。

mysql> INSERT INTO animals (id,name) VALUES(-50,'-middle');
Query OK, 1 row affected (0.00 sec) 
mysql> select * from animals;
+-----+-----------+
| id  | name      |
+-----+-----------+
| -50 | -middle   |
|   1 | fish       |
|   2 | cat       |
|   3 | penguin   |
|   4 | lax       |
|   5 | whale     |
|   6 | ostrich   |
|   7 | groundhog |
|   8 | squirrel  |
|  50 | middle    |
| 100 | rabbit    |
+-----+-----------+
11 rows in set (0.00 sec) 
mysql> show create table animals;
+---------+--------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                     |
+---------+--------------------------------------------------------------------------------------------------+
| animals | CREATE TABLE `animals` (  `id` mediumint NOT NULL AUTO_INCREMENT,  
`name` char(30) NOT NULL,  PRIMARY KEY (`id`)) ENGINE=InnoDB AUTO_INCREMENT=101 DEFAULT CHARSET=utf8         |
+---------+--------------------------------------------------------------------------------------------------+

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