title: When not to use surrogate primary keys?
body: <p>I have  several database tables that just contain a single column and very few rows, often just an ID of something defined in another system. These tables are then referenced with foreign keys from other tables. For example one table contains country codes (SE, DK, US etc). All values are always unique natural keys and they are used as primary keys in other (legacy) systems.</p>
  
  <p>It seems really unnecessary to introduce a new surrogate key to these tables, or?</p>
  
  <p>In general, what are the exceptional cases when surrogate keys shouldn't be used?</p>