Innodb中自增长值的列

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 key
mysql> 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>