mysql> create table zc_test2(id int);Query OK, 0 rows affected (1.37 sec)mysql> insert into zc_test2 values (10),(9),(8),(7),(6);Query OK, 5 rows affected (0.04 sec)Records: 5 Duplicates: 0 Warnings: 0mysql> select * from zc_test2;+------+| id |+------+| 10 || 9 || 8 || 7 || 6 |+------+5 rows in set (0.00 sec)mysql> ALTER TABLE zc_test2 add id2 int;Query OK, 0 rows affected (2.84 sec)Records: 0 Duplicates: 0 Warnings: 0mysql> select * from zc_test2;+------+------+| id | id2 |+------+------+| 10 | NULL || 9 | NULL || 8 | NULL || 7 | NULL || 6 | NULL |+------+------+5 rows in set (0.00 sec)mysql> alter table zc_test2 change id2 id2 int NOT NULL AUTO_INCREMENT;ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a keymysql> alter table zc_test2 change id2 id2 int NOT NULL AUTO_INCREMENT primary key;Query OK, 5 rows affected (1.34 sec)Records: 5 Duplicates: 0 Warnings: 0mysql> select * from zc_test2;+------+-----+| id | id2 |+------+-----+| 10 | 1 || 9 | 2 || 8 | 3 || 7 | 4 || 6 | 5 |+------+-----+5 rows in set (0.00 sec)
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