Primary Key and Non-Keys

Entities and tables are drawn as a box with a horizontal line near the top of the box.

The area above the horizontal line is known as the key area because that is where the entity's primary key resides. The primary key can be comprised of one or more attributes that uniquely identify the entity.

The area below the horizontal line is known as the non-key area because that is where all other attributes reside.

Note: A foreign key may appear in either the primary key or the non-key area of an entity or table. For more information about foreign keys, see Foreign Key.