Alternate key

A candidate key which is not chosen as a primary key is an alternate key. That was shown in the table Purchase

If you select Serial# as the primary key then the Itemcode is the alternate key. It is very important that you understand that a primary key is the only sure way to identify the rows of a table. Therefore, an alternate key should have the value NULL. NULL value is not to be permitted in a primary key since it would be hard to uniquely identify rows containing NULL values.

Purchase