Algo como:
ORDER BY CASE
WHEN Column1 IS NOT NULL THEN Column1
ELSE Column2
END
O mesmo que escrever:
ORDER BY COALESCE(Column1, Column2)
Ambos devem funcionar em qualquer RDBMS sensato.
ORDER BY CASE
WHEN Column1 IS NOT NULL THEN Column1
ELSE Column2
END
ORDER BY COALESCE(Column1, Column2)