title: Any difference between union all corresponding and outer union corresponding?
body: <p>Could someone verify my understanding of proc sql union operations? My interpretation of the differences between outer union and union is the following:</p>
  
  <ol>
  <li><p>Union deletes duplicate rows, while outer union does not</p></li>
  <li><p>Union will overlay columns, while outer union, by default, will not.</p></li>
  </ol>
  
  <p>So, would there be any difference between union all corresponding and outer union corresponding? It seems like "ALL" would remove the first difference, and "CORRESPONDING" would remove the second difference, but I'm concerned there could be an additional difference between the two I'm not seeing.</p>