select bank_code, cu_code, cash_buy, cash_sell,
       (CASE WHEN cash_buy = 0 THEN null ELSE cash_buy END) AS cash_buy1 
FROM bank_currency_info_latest 
WHERE cu_code='CNY' 
ORDER BY ISNULL(cash_buy1), cash_buy1



SELECT * 
FROM test 
ORDER BY (case WHEN ID IS NULL THEN 1 ELSE 0 END), ID ASC