Innodb中，自增长值的列必须是索引，同时必须是索引的第一个列。如果不是第一个列，数据库会报出异常

mysql> create table t_inc01(    -> a int auto_increment,    -> b int,    -> key(b,a)    -> ) engine=innodb;ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a keymysql> create table t_inc01(    -> a int auto_increment,    -> b int,    -> key(a,b)    -> )engine=innodb;Query OK, 0 rows affected (0.01 sec)mysql>

 

MyISAM引擎中没有这个问题

mysql> create table t_inc02(    -> a int auto_increment,    -> b int,    -> key (b,a)    -> )engine=myisam;Query OK, 0 rows affected (0.00 sec)mysql>