--对于innodb表，自动增长列必须是索引。如果是组合索引，也必须是前导列。
mysql> create table innodb_auto
-> ( id1 int not null auto_increment,
-> id2 int not null,
-> val varchar(10),
-> index(id2,id1)
-> ) engine=innodb;
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key
--但是对于myisam表，自动增长列可以是组合索引的其他列。这样插入记录后，自动增长列是按照组合索引的前面几列进行排序后递增的。如下实验：
--创建一个myisam表
mysql> create table myisam_auto
-> ( id1 int not null auto_increment,
-> id2 int not null,
-> val varchar(10),
-> index(id2,id1)
-> ) engine=myisam;
Query OK, 0 rows affected (0.11 sec)
mysql> insert into myisam_auto(id2,val) values(2,'2'),(3,'3'),(4,'4'),(5,'5'),(2,'2'),(3,'3'),(4,'4');
Query OK, 7 rows affected (0.00 sec)
Records: 7 Duplicates: 0 Warnings: 0
mysql> select * from myisam_auto;
+-----+-----+------+
| id1 | id2 | val |
+-----+-----+------+
| 1 | 2 | 2 |
| 1 | 3 | 3 |
| 1 | 4 | 4 |
| 1 | 5 | 5 |
| 2 | 2 | 2 |
| 2 | 3 | 3 |
| 2 | 4 | 4 |
+-----+-----+------+
7 rows in set (0.00 sec)
mysql> insert into myisam_auto(id2,val) values(3,'3'),(4,'4');
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from myisam_auto;
+-----+-----+------+
| id1 | id2 | val |
+-----+-----+------+
| 1 | 2 | 2 |
| 1 | 3 | 3 |
| 1 | 4 | 4 |
| 1 | 5 | 5 |
| 2 | 2 | 2 |
| 2 | 3 | 3 |
| 2 | 4 | 4 |
| 3 | 3 | 3 |
| 3 | 4 | 4 |
+-----+-----+------+
9 rows in set (0.00 sec)
由以上实验可以看出对于组合索引 id1，id2 插入一些数据，自动组合列是根据组合索引的第一列d2 进行排序后递增的。
