SELECT 
 LandIdent,
 Land,
 LandKurzbezeichnung,
 SRF =
  CASE 
  WHEN SRF IS NULL THEN 255
  WHEN SRF = 0     THEN 255  
  ELSE SRF 
 END
FROM tblLaender
WHERE (Sperre = 0 OR Sperre IS NULL)
UNION
SELECT 
 0, 
 '-- Bitte auswählen --',
 '',
 0 
ORDER BY 
SRF ASC,
Land ASC