title: Best way to write union query when dealing with NULL and Empty String values
body: <p>I have to write  a query that performs a union between two tables with similar data.  The results need to be distinct.  The problem I have is that some fields that should be the same are not when it comes to empty values.  Some are indicated as null, and some have empty string values.  My question is, is there a better way to perform the following query? (without fixing the actual data to ensure proper defaults are set, etc) Will using the Case When be a big performance hit?</p>
  
  <pre><code>Select  
      When Column1 = '' Then NULL Else Column1 as [Column1],
      When Column2 = '' Then NULL Else Column2 as [Column2]
  From TableA
  
  UNION ALL
  
  Select 
      When Column1 = '' Then NULL Else Column1 as [Column1],
      When Column2 = '' Then NULL Else Column2 as [Column2]
  From TableB
  </code></pre>