Sliding window functions in SQL Server, advanced calculation - sql

Question

I have a problem that it's very easy to be solved in C# code for example, but I have no idea how to write in a SQL query.
Here is the situation: let's say I have a table with 3 columns (ID, Date, Amount), and here is some data:
ID Date Amount
-----------------------
1 01.01.2016 -500
2 01.02.2016 1000
3 01.03.2016 -200
4 01.04.2016 300
5 01.05.2016 500
6 01.06.2016 1000
7 01.07.2016 -100
8 01.08.2016 200
The result I want to get from the table is this (ID, Amount .... Order By Date):
ID Amount
-----------------------
2 300
4 300
5 500
6 900
8 200
The idea is to distribute the amounts into installments, but the thing is when negative amount comes into play you need to remove amount from the last installment. I don't know how clear I am, so here is an example:
Let's say I have 3 Invoices with amounts 500, 200, -300.
If i start distribute these Invoices, first i distribute the amount 500, then 200. But when i come to the third one -300, then i need to remove from the last Invoice. In other workds 200 - 300 = -100, so the amount from second Invoice will disappear, but there are still -100 that needs to be substracted from first Invoice. So 500 - 100 = 400. The result i need is dataset with one row (first invoice with amount 400)
Another example when the first invoice is with negative amount (-500, 300, 500).
In this case, the first (-500) invoice will make the second disappear and another 200 will be substracted from the third. So the result will be: Third Invoice with amount 300.
This is something like Stack implementation in programming language, but i need to make it with sliding-window functions in SQL Server.
If anyone have any idea, please share.
Thanks.

Answer 1

I solved it using TSQL. But I think what this task also can solve using recursive CTE.
I used ID for finding a prev or next row.
-- create and fill test table
CREATE TABLE Invoices(
ID int,
[Date] date,
Amount float
)
INSERT Invoices(ID,Date,Amount) VALUES
(1,'20160101', -500),
(2,'20160201', 1000),
(3,'20160301', -200),
(4,'20160401', 300),
(5,'20160501', 500),
(6,'20160601', 1000),
(7,'20160701', -100),
(8,'20160801', 200)
My solution
-- copy all the data into temp table
SELECT *
INTO #Invoices
FROM Invoices
DECLARE
#nID int,
#nAmount float,
#pID int
-- run infinity loop
WHILE 1=1
BEGIN
-- set all the variables to NULL
SET #nID=NULL
SET #nAmount=NULL
SET #pID=NULL
-- get data from the last negative row
SELECT
#nID=ID,
#nAmount=Amount
FROM
(
SELECT TOP 1 *
FROM #Invoices
WHERE Amount<0
ORDER BY ID DESC
) q
-- get prev positive row
SELECT #pID=ID
FROM
(
SELECT TOP 1 *
FROM #Invoices
WHERE ID<#nID
AND Amount>0
ORDER BY ID DESC
) q
IF(#pID IS NULL)
BEGIN
-- get next positive row
SELECT #pID=ID
FROM
(
SELECT TOP 1 *
FROM #Invoices
WHERE ID>#nID
AND Amount>0
ORDER BY ID
) q
END
-- exit from loop
IF(#pID IS NULL) BREAK
-- substract amount from positive row
UPDATE #Invoices
SET
Amount+=#nAmount
WHERE ID=#pID
-- delete used negative row
DELETE #Invoices
WHERE ID=#nID
END
-- show result
SELECT *
FROM #Invoices
DROP TABLE #Invoices