SQL-行から枯渇した値を減算する

OPの演習として残しました：サンプルデータを指定して正しい結果を把握し、次のクエリの結果を要約します：

-- Create some test data.
declare @Pooled_Lots as table ( Id int, Pool int, Lot int, Quantity int );
insert into @Pooled_Lots ( Id, Pool, Lot, Quantity ) values
  ( 1, 1, 1, 5 ), ( 2, 1, 2, 10 ), ( 3, 1, 3, 4 ),
  ( 4, 2, 1, 7 ),
  ( 5, 3, 1, 1 ), ( 6, 3, 2, 5 );
declare @Pool_Consumption as table ( Id int, Pool int, QuantityConsumed int );
insert into @Pool_Consumption ( Id, Pool, QuantityConsumed ) values
  ( 1, 1, 17 ), ( 2, 2, 8 ), ( 3, 3, 10 );

select * from @Pooled_Lots order by Pool, Lot;
select * from @Pool_Consumption order by Pool;

with Amos as (
  -- Start with Lot 1 for each Pool.
  select PL.Pool, PL.Lot, PL.Quantity, PC.QuantityConsumed,
    case
      when PC.QuantityConsumed is NULL then PL.Quantity
      when PL.Quantity >= PC.QuantityConsumed then PL.Quantity - PC.QuantityConsumed
      when PL.Quantity < PC.QuantityConsumed then 0
      end as RunningQuantity,
    case
      when PC.QuantityConsumed is NULL then 0
      when PL.Quantity >= PC.QuantityConsumed then 0
      when PL.Quantity < PC.QuantityConsumed then PC.QuantityConsumed - PL.Quantity
      end as RemainingDemand
    from @Pooled_Lots as PL left outer join
      @Pool_Consumption as PC on PC.Pool = PL.Pool
    where Lot = 1
  union all
  -- Add the next Lot for each Pool.
  select PL.Pool, PL.Lot, PL.Quantity, CTE.QuantityConsumed,
    case
      when CTE.RunningQuantity + PL.Quantity >= CTE.RemainingDemand then CTE.RunningQuantity + PL.Quantity - CTE.RemainingDemand
      when CTE.RunningQuantity + PL.Quantity < CTE.RemainingDemand then 0
      end,
    case
      when CTE.RunningQuantity + PL.Quantity >= CTE.RemainingDemand then 0
      when CTE.RunningQuantity + PL.Quantity < CTE.RemainingDemand then CTE.RemainingDemand - CTE.RunningQuantity - PL.Quantity
      end
    from Amos as CTE inner join
      @Pooled_Lots as PL on PL.Pool = CTE.Pool and PL.Lot = CTE.Lot + 1
  )
select *,
  case
    when Lot = ( select max( Lot ) from @Pooled_Lots where Pool = Amos.Pool ) then RunningQuantity - RemainingDemand
    else NULL end as SurplusOrDeficit
  from Amos
  order by Pool, Lot;