testdb> create table foo(id int unsigned not null, bar varchar(32), primary key(id));
Query OK, 0 rows affected (0.01 sec)

testdb> insert into foo(bar) values('foo');
Query OK, 1 row affected, 1 warning (0.00 sec)

testdb> insert into foo(bar) values('bar');
ERROR 1062 (23000): Duplicate entry '0' for key 1