create table t1(a int(10)unsigned not null auto_increment primary key,
b varchar(255) not null) engine=innodb default charset=utf8;
insert into t1 values(1,'aaa'),(2,'bbb');
alter table t1 auto_increment=1;
insert into t1 values(NULL, 'ccc');
select * from t1;
a	b
1	aaa
2	bbb
3	ccc
drop table t1;