A field in a database table identified as of importance is designated a primary key. There can be only one primary key in a table. A primary key field must have unique entries for each record in the table. A primary key field also may not be blank in any of the records in a table. A foreign key is a field with the same name as a primary key that is located in a different table. Foreign keys are identified automatically when primary keys are designated in other tables attached to the same vector/CAD object. Primary and foreign key relationships between tables let you establish computed fields that incorporate values from more than one table in their defining expression.