Order by Column1 if Column1 is not null, otherwise order by Column2

Try this

  ORDER BY COALESCE(fieldA, fieldB);

Something like:

ORDER BY CASE 
    WHEN Column1 IS NOT NULL THEN Column1
    ELSE Column2
END

Or the corresponding syntactic sugar:

ORDER BY COALESCE(Column1, Column2)

Note that the datatype of the two columns must be comparable (the RDBMS defines the rules).

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