乘以代码

Question

我的查询没有结果，我不明白：

(
  select checkinkilometer 
  from rentobjtransact 
  where rentcontract.rentcontractid = rentobjtransact.rentcontractid 
  and RentObjTransact.isActive = 1
) *
(
  case when RentObj.isAlienRentObj = 0 
    then TO_CHAR(car.CO2Emission, '999999') 
    else rentobj.ClassificDescr3
  end
) - 
(
  select checkoutkilometer 
  from rentobjtransact 
  where rentcontract.rentcontractid = rentobjtransact.rentcontractid 
  and RentObjTransact.isActive = 1
) *
(
  case when RentObj.isAlienRentObj = 0
    then TO_CHAR(car.CO2Emission, '999999') 
    else rentobj.ClassificDescr3 
  end
) as "CO2 verbruik"

当我获取部分代码时，我得到的结果是正确的：

(
  select checkinkilometer 
  from rentobjtransact 
  where rentcontract.rentcontractid = rentobjtransact.rentcontractid 
  and RentObjTransact.isActive = 1
) -
(
  select checkoutkilometer 
  from rentobjtransact 
  where rentcontract.rentcontractid = rentobjtransact.rentcontractid 
  and RentObjTransact.isActive = 1
) as "km"

还有，

(
  case when RentObj.isAlienRentObj = 0 
    then TO_CHAR(car.CO2Emission, '999999') 
    else rentobj.ClassificDescr3 
  end
) as "CO2"

是对的。但是当我想要组合代码时，我得不到任何结果。请给我一个提示？