title: Combine two tables for one output
body: <p>Say I have two tables:</p>
  
  <p>KnownHours:</p>
  
  <pre>
  ChargeNum    CategoryID    Month    Hours
  111111       1             2/1/09   10
  111111       1             3/1/09   30
  111111       1             4/1/09   50
  222222       1             3/1/09   40
  111111       2             4/1/09   50
  </pre>
  
  <p>UnknownHours:</p>
  
  <pre>
  ChargeNum   Month   Hours
  111111      2/1/09  70
  111111      3/1/09  40.5
  222222      7/1/09  25.5
  </pre>
  
  <p>I need to group these hours, ignoring Month, into a single data table so that my expected result is the following:</p>
  
  <pre>
  ChargeNum    CategoryID     Hours
  111111       1              90
  111111       2              50
  111111       Unknown        110.5
  222222       1              40
  222222       Unknown        25.5
  </pre>
  
  <p>I cannot seem to figure this out. Any help would be greatly appreciated!</p>
  
  <p>EDIT: I need to sum the hours for each ChargeNum/Category combination.  I updated the sample data to reflect this.</p>