title: SQL: Aggregate function returning 0's when using WHERE filter
body: <p>I have a table with 10 columns listing results of a marketing campaign including person name, date, residence area, request, and response. </p>
  
  <p>The request column is always a number, indicating the number of survey requests we've sent a specific person.</p>
  
  <p>The response column is a number from 0 to X, representing the number of surveys a person has responded to take. </p>
  
  <p>Now, when I look through the actual table, there are probably around 10% response rate. With lots of non-zero entries in the response column. </p>
  
  <p>However, when I write an aggregate function like this: </p>
  
  <pre><code>SELECT person, date, SUM(requests), SUM(response)
  FROM analytics.SurveyResults0304
  WHERE group_type = 'Youth'
  GROUP BY person, date; 
  </code></pre>
  
  <p>I get a correct number for SUM(requests), but I get a big fat "0"  for SUM(response)? 
  It's the same for all 3 group types. It returns a 0 for SUM(response)</p>
  
  <p>Update: it works fine when I don't include the group_type filter, but how come I can't use it with the WHERE filter?</p>
  
  <p>Thanks!!!</p>
  
  <p>EDIT2: Sample Table</p>
  
  <pre><code>  Person    Date       Group_Type   Requests Response  Neighborhood FirstName  
    --------  --------   -----------  -------- --------  ------------ ---------
    Nixon     3/3/2013    Youth          3        3  Chinatown    Richard
    Clinton   3/3/2013    Youth          4        0  Gunhill      Bill
    Mao       3/3/2013    Youth          5        0  Berryville   Chairman
    Nixon     3/4/2013    Youth         17        2  Townsford    Richard
    Gates     3/3/2013    Elderly       41        5  Chinatown    Bill
    Gates     3/4/2013    Elderly        0        0  Chinatown    Bill
    Gates     3/5/2013    Elderly        0        0  Chinatown    Bill
    Gates     3/6/2013    Elderly        0        0  Chinatown    Bill
  </code></pre>
  
  <p>For example
  When I do: </p>
  
  <pre><code>SELECT  SUM(requests), SUM(response)
  FROM analytics.SurveyResults0304
  WHERE group_type = 'Youth';
  </code></pre>
  
  <p>It returns 70 for request, and 0 for response across the board. </p>