Question

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?

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

Answer 1

I don’t think it would make any difference in performance, but NULLIF is another way to write this and, IMHO, looks a little cleaner.

Select  
    NULLIF(Column1, '') as [Column1],
    NULLIF(Column2, '') as [Column2]
From TableA

UNION

Select 
    NULLIF(Column1, '') as [Column1],
    NULLIF(Column2, '') as [Column2]
From TableB