SELECT 
  c1.currency AS [From], c2.currency AS [To] , cer.Exchange_Rate
FROM
  currency c1 JOIN currency c2 ON c1.rec_id <> c2.rec_id
  LEFT OUTER JOIN CurrencyExchangeRate cer ON c1.rec_id = cer.[from] 
         AND c2.rec_id = cer.[to]