title: Coalesce values in one column based on another column
body: <p>How do I coalesce values in a column grouped by another column? </p>
  
  <p>e.g. How do I coalesce column 2 values by column 1 values? So turn the following:</p>
  
  <pre><code>Column 1|Column 2
  -----------------
  A       |Bob
  A       |
  B       |Mary
  C       |
  C       |Kevin
  C       |
  D       |
  D       |
  </code></pre>
  
  <p>Into this:</p>
  
  <pre><code>Column 1|Column 2
  -----------------
  A       |Bob
  B       |Mary
  C       |Kevin
  D       |
  </code></pre>
  
  <p>Thanks!</p>