When you assign one field as a primary key you can use auto-increment attribute to that field. It generates unique identity for the row inserting a unique auto increment value. You can also reset auto increment values.
For example if a table has 5 rows and you insert another row then the automatic value for the new row would be 6.
Using the ALTER TABLE statement you can reset auto increment value.
ALTER TABLE tbl_name AUTO_INCREMENT = 5;
Also truncating a table reset auto increment to zero.
TRUNCATE TABLE tbl_name;
