title: Get the difference between two values
body: <p>I got totally stuck on comparing two tables and getting the difference between them</p>
  
  <p>So here we go:
  I got table a with the following columns
  Name|Value|Date</p>
  
  <p>and the second table b with the same columns</p>
  
  <p>What i wanna do now is get the difference between the values like</p>
  
  <p>Table a</p>
  
  <p>Name|Value|Date</p>
  
  <p>Test|3|2013-20-06</p>
  
  <p>Table b</p>
  
  <p>Name|Value|Date</p>
  
  <p>Test|9|2013-20-06</p>
  
  <p>What i wann get is the difference between the 3 and 9 so i would recieve 6</p>
  
  <p>Any Idea how i'm able to get that from a query in my PostgreSQL-DB?</p>