I have a MySQL table with an auto increment primary key. I deleted some rows in the middle of the table. Now I have, for example, something like this in the ID column: 12, 1
You could drop the primary key column and re-create it. All the ids should then be reassigned in order.
However this is probably a bad idea in most situations. If you have other tables that have foreign keys to this table then it will definitely not work.