title: Primary Key Generated From External System
body: <p>What are the general guidelines for the following scenario:</p>
  
  <p>You need to create a table where the PK is a value that is generated from an external system. It is unique, similar to using a SSN. </p>
  
  <p>Does it make sense for this to be the primary key? Especially considering it will be used in foreign key relationships with two other tables. Or, would it be better to create an auto-incrementing ID field in addition to the unique key, and use the auto-incrementing ID in the table relationships?</p>