title: SQL selecting rows by most recent date with two unique columns
body: <p>Using the following query and results, I'm looking for the most recent entry where the ChargeId and ChargeType are unique.</p>
  <pre><code>select chargeId, chargeType, serviceMonth from invoice
  
      CHARGEID    CHARGETYPE  SERVICEMONTH
  1   101         R           8/1/2008
  2   161         N           2/1/2008
  3   101         R           2/1/2008
  4   101         R           3/1/2008
  5   101         R           4/1/2008
  6   101         R           5/1/2008
  7   101         R           6/1/2008
  8   101         R           7/1/2008
  </code></pre>
  <p>Desired:</p>
  <pre><code>    CHARGEID    CHARGETYPE  SERVICEMONTH
  1   101         R           8/1/2008
  2   161         N           2/1/2008
  </code></pre>