Primary Key

A primary key is the field or fields in a table that uniquely identifies the record.  An everyday example of a primary key is a person's social security number in the United States.  

Microsoft® Access, as well as other databases, allow you to identify a field or fields as the primary key.  The database then will insure that the field(s) you select are unique and do not contain a null value (missing or unknown data).  If you try to enter a duplicate value, an error is generated.

Consider the previous table example:

No single field would be suitable as a primary key, except perhaps the Email field.  However, if the Email field were used as the primary key, errors may still be generated.  Since the Email field is a text field where user-entry is allowed, user-entry errors are always possible.  

The best practice for assigning a primary key in a table is to create an AutoNumber field.  The database automatically assigns a new unique number for every new record.  In the previous example, the table would be modified to include an AutoNumber field as shown:

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