title: Unique values with string_agg on more than 1 column
body: <p>I am trying to group by and get list of values for multiple columns. Here is an example:</p>
  
  <pre><code>City   | State | Income
  -------+-------+--------
  Salem  |  OH   | 40000
  Salem  |  OH   | 45000
  Mason  |  OH   | 50000
  Dayton |  OH   | 60000
  Salem  |  MA   | 40000
  Mason  |  MA   | 45000
  Mason  |  MA   | 50000
  Dayton |  MA   | 70000
  Salem  |  PA   | 45000
  Mason  |  PA   | 50000
  Dayton |  PA   | 60000
  </code></pre>
  
  <p>The result I am looking for is:</p>
  
  <pre><code>City   |  States    | Income
  -------+------------+--------------
  Salem  | OH,MA,PA   | 40000,45000
  Mason  | OH,MA,PA   | 50000,45000
  Dayton | OH,MA,PA   | 60000,70000
  </code></pre>
  
  <p>I managed to get this far:</p>
  
  <pre><code>City   |  States    | Income
  -------+------------+-------------------------
  Salem  | OH,MA,PA   | 40000,40000,45000,45000
  Mason  | OH,MA,PA   | 50000,50000,50000,45000
  Dayton | OH,MA,PA   | 60000,70000,60000
  </code></pre>
  
  <p>How do I go from here to the result set?</p>
  
  <pre><code>City   |  States    | Income
  -------+------------+-------------------------
  Salem  | OH,MA,PA   | 40000,45000,50000
  Mason  | OH,MA,PA   | 50000,45000
  Dayton | OH,MA,PA   | 60000,70000
  </code></pre>