Window Functions - Running Total with reset

Question

I am using SQL Server 2012 to build an inventory planning / reorder engine.

I have a bunch of dated transactions, call them credits and debits. I want to do two thi

Answer 1

Ugh, based on your comments, the only thing I can think to do is use a cursor, which I hate doing.

SQL Fiddle

declare @Date date
declare @Qty int
declare @RR int


declare @running int  = 0

declare @results table
(dt date,
 qty int,
 rt int,
 rr int
)

declare C cursor for
select TDate, Qty,
RecommendedReplenish 
from (
    select 
        TDate, 
        Qty,
        -1 * (CASE WHEN Qty < 0 AND SUM(Qty) OVER (ORDER BY TDate ROWS UNBOUNDED     PRECEDING) < 0 
                THEN 
            CASE WHEN Qty >  SUM(Qty) OVER (ORDER BY TDate ROWS UNBOUNDED PRECEDING)     THEN Qty ELSE SUM(Qty) OVER (ORDER                        BY TDate ROWS UNBOUNDED PRECEDING) END
        ELSE 0 END) as RecommendedReplenish
        /* Wrong, does not account for balance resetting to zero */
    from TX 
) T order by TDate

open c
fetch next from c into @date,@qty,@rr
WHILE @@FETCH_STATUS = 0
BEGIN



  set @running = @running + @qty
  if @running <0
    begin
      set @running = 0
    end

  insert into @results values (@date,@qty,@running,@rr)

  fetch next from c into @date,@qty,@rr
end
close c
deallocate c
select
*
from @results

Which as far as I can tell, gives you the desired result. It ain't pretty, I'm sure it could use some cleanup, but it works.

+-------------+------+-----+----+
|     DT      | QTY  | RT  | RR |
+-------------+------+-----+----+
| 2014-03-01  |  20  | 20  |  0 |
| 2014-03-02  | -10  | 10  |  0 |
| 2014-03-03  | -20  |  0  | 10 |
| 2014-03-04  | -10  |  0  | 10 |
| 2014-03-05  |  30  | 30  |  0 |
| 2014-03-06  | -20  | 10  | 10 |
| 2014-03-07  |  10  | 20  |  0 |
| 2014-03-08  | -20  |  0  | 20 |
| 2014-03-09  |  -5  |  0  |  5 |
+-------------+------+-----+----+

Answer 2

Using a temp-table you could apply the Replenishment as you go. Not sure if it would be much faster than the cursor approach from @Andrew; probably depends on how often the RT dips below zero. I used a simple subquery to calculate the RT, less typing, same result although I agree it takes an extra step.

SQL Fiddle

CREATE TABLE TX (TDate DATETIME, Qty   INT, Replenish INT NULL, RT INT NULL);

INSERT INTO TX VALUES ('2014-03-01', 20, NULL, NULL);  
INSERT INTO TX VALUES ('2014-03-02',-10, NULL, NULL); 
INSERT INTO TX VALUES ('2014-03-03',-20, NULL, NULL); 
INSERT INTO TX VALUES ('2014-03-04',-10, NULL, NULL); 
INSERT INTO TX VALUES ('2014-03-05', 30, NULL, NULL); 
INSERT INTO TX VALUES ('2014-03-06',-20, NULL, NULL);  
INSERT INTO TX VALUES ('2014-03-07', 10, NULL, NULL);  
INSERT INTO TX VALUES ('2014-03-08',-20, NULL, NULL); 
INSERT INTO TX VALUES ('2014-03-09', -5, NULL, NULL);
GO

-- calculate (real) running-totals
UPDATE TX 
   SET RT = (SELECT SUM(p.Qty)
               FROM TX p
              WHERE p.TDate <= upd.TDate)
  FROM TX upd
GO

-- create a loop to find if there are negative RT's and fix them untill there are none left
DECLARE @below_zero_date DATETIME,
        @below_zero_value INT

-- SELECT * FROM TX ORDER BY TDate

SELECT @below_zero_value = NULL
SELECT TOP 1 @below_zero_date = TDate,
             @below_zero_value = RT
  FROM TX
 WHERE RT < 0
 ORDER BY TDate

WHILE @below_zero_value IS NOT NULL
    BEGIN
        UPDATE TX
           SET RT = RT - @below_zero_value,
               Replenish = (CASE TDate WHEN @below_zero_date THEN - @below_zero_value ELSE NULL END)
         WHERE TDate >= @below_zero_date

        -- SELECT * FROM TX ORDER BY TDate

        SELECT @below_zero_value = NULL

        SELECT TOP 1 @below_zero_date = TDate,
                     @below_zero_value = RT
          FROM TX
         WHERE RT < 0
           AND TDate > @below_zero_date
         ORDER BY TDate
    END

SELECT * FROM TX ORDER BY TDate

UPDATE: added AND TDate > @below_zero_date as (minor) improvement; it will only have a significant effect when there is 'quite a bit' of data in the table.

Answer 3

This can be done using a set-based solution:

1.Compute the normal running total (call it RT)

2.Compute the running minimum of RT (call it MN)

When MN is negative, -MN is the total quantity you had to replenish so far. Let replenish_rt be -MN when MN is negative. So, the new running total (call it new_rt) is rt + replenish_rt. And if you need to return the current replenish quantity needed, subtract the pervious replenish_rt (using LAG) from the current.

Here's the complete solution query:

with c1 as
(
  select *,
    sum(qty) over(order by tdate rows unbounded preceding) as rt
  from tx
),
c2 as
(
  select *,
    -- when negative, mn is the total qty that had to be
    -- replenished until now, inclusive
    min(rt) over(order by tdate rows unbounded preceding) as mn_cur
  from c1
)
select tdate, qty, rt,
  replenish_rt - lag(replenish_rt, 1, 0) over(order by tdate) as replenish,
  rt + replenish_rt as new_rt
from c2
  cross apply(values(case when mn_cur < 0 then -mn_cur else 0 end)) as a1(replenish_rt);

Cheers, Itzik