Every table must have at least one field that has a unique value for each record. For instance, in a table of the dogs your kennel owns, you might assign an ID number to each dog, and have an ID # field in your table. Or you might choose to use each dog's AKC (American Kennel Club) registration number. This unique identifier field is known as the Primary Key field.
You must tell Access which field you are going to use as the Primary Key, so it can prevent you from accidentally entering the same value for more than one record in that field. To set a primary key, follow these steps:
In Table Design view, select the field that you want for the primary key.
Select EditPrimary Key, or click the Primary Key button on the ...