A primary key is a unique value identifying the record.
The primary key is unique within the table. It is also case insensitive, it does not distinguished between lowercase and uppercase letters.
You cannot change the primary key once it is entered. If you make an incorrect entry, or if you want to change it for some other reason, you must select and delete the record. If you attempt to create a new row with the same primary key as an existing one, an error message is displayed.