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

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.