Learn how to create an Entity Relationship Diagram with Primary Keys, Foreign Keys, and Composite Keys in this advanced ERD tutorial. We provide step-by-step training on how to identify these different keys, as well as helpful information on bridge tables, data types, and how your ER Diagram relates to a database management system. A Primary Key is an attribute (or field) that uniquely identifies every record in a certain table. There’s one Primary Key per entity, and they must be unique, never-changing, and never-null. You note a Primary Key in your Entity Relationship Diagram with “PK” to the left of the appropriate attribute. A Foreign Key is an attribute in one entity that links to the Primary Key of another entity. Consequently, they enable relationships between those entities. There can be multiple foreign keys per entity, or none at all. You note a Foreign Key in your ERD with “FK” to the left of the appropriate attributes. —