title: What could be the purpose of primary keys on all tables being derived from a single table?
body: <p>I've really been scratching my head over this and don't know how to ask the question well enough to find an answer on Google or StackOverflow etc.</p>
  
  <p>There is a very old system used at work - I don't have access to the server side so can't view its tables, but I do know its an SQL database and have done enough experimenting with the API to see what adding to each table does, and I'm questioning how it allocates primary keys;</p>
  
  <p>It has a lot of tables, each with a primary key as expected, but the primary key on any/all of its tables seems to be allocated so that there is absolutely no duplication of primary keys anywhere in the system.</p>
  
  <p>e.g.</p>
  
  <pre><code>add row to table 1 get pk = 1
  add row to table 2 get pk = 2
  add row to table 1 again, get pk = 3
  add row to table 10 and get pk = 4
  </code></pre>
  
  <p>Is this method some sort of old database technique?</p>
  
  <p>What could be the purpose of doing this?</p>
  
  <p>There are more funny nuances that I won't get into detail of, e.g. a certain range of pk's being allocated for certain tables but I just wanted to see if anyone recognises the main principle here and if there's a point to it, or if it's just bad / weird design</p>