Modify column to auto_increment

[Aigini Navaneethan]

Hi,

I want to modify an existing table column to auto-increment, and I have tried these but I get error :

mysql> ALTER TABLE lz_chat_archive_dup2 MODIFY COLUMN chat_id INT(10) AUTO_INCREMENT;
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key

mysql> ALTER TABLE lz_chat_archive_dup2 MODIFY COLUMN chat_id INT(10) AUTO_INCREMENT KEY;
ERROR 1265 (01000): Data truncated for column 'chat_id' at row 12

How do I resolve this?