title: Set multiple "primary keys" on one AccountID
body: <p>This seems like a simple problem. However, I'm blocked.</p>
  
  <p>I have 6 Product ID's: <code>1001</code>, <code>1002</code>, <code>1003</code>, <code>1004</code>, <code>1005</code>, <code>1006</code>.</p>
  
  <p>In a table, I also have a list of account IDs. Each Account ID can have one of many product IDs(1-6), but no duplicates. </p>
  
  <p>How do I add a constraint to allow multiple AccountID's in a table that could up to 1-6 product ID's but none of the productID's per one account could be duplicated? </p>
  
  <p>Example Table :</p>
  
  <p>Good Table</p>
  
  <pre><code>Account_ID  ProductID
  138777  100004
  138777  100003
  --------------
  21252   100003
  138778  100003
  138779  100004
  138780  100003
  138781  100001
  138782  100004
  </code></pre>
  
  <p>Bad table</p>
  
  <pre><code>Account_ID  ProductID
  138777  100003
  138777  100003
  ---------------
  21252   100003
  138778  100003
  138779  100004
  138780  100003
  138781  100001
  138782  100004
  </code></pre>
  
  <p>What's the most logical way of implementing this constraint? </p>