title: Unique serial ID over all subtypes
body: <p>I have the following problem with designing a database:
  In my EER-diagram I have the supertype Entity A with 2 subtype entities B and C.</p>
  
  <p>The table-schemas look like this:</p>
  
  <pre><code>Create table A {
       attributes of A here,
       ID int
  }
  
  Create table B {
       attributes of B here,
      ID int SERIAL
  }
  
  Create table C {
       attributes of C here,
      ID int SERIAL
  </code></pre>
  
  <p>ID in table B and C refer to table A to maintain the supertype-relation.
  I have a list of data which contains all attributes except for the ID which I have to make on my own while reading in the data (using the "create table as"-statement).
  I have this problem: When I read in the data, the ID attributes of B and C all start counting from 1 to ... .
  Thought I want the ID's of B and C to be different.</p>
  
  <p>I tried putting the "SERIAL"-keyword in table A, but then is my question: How can I get that ID to B or C?</p>