SQL query to calculate interval discount - sql

Question

I have trouble understanding how I can solve this problem with a T-SQL query.
I have a price column and a volume column. In another table a have discounts at different levels of volume. So my discount table could have values as
(StartLevel, DiscountFactor)
(0, 1);
(25, 0.95);
(50, 0.90);
(100, 0.75)
What I want is to calculate a total price. If Volume is 35, I want it to multiply
Price x ((35-25) x 0.95 + (25-0) x 1)
If the volume is 200, it should be
Price x ((200-100) x 0.75 + (100-50) x .9+(50-25) x .95+(25) x 1)
Can anybody help me with a query that solves this?

Answer 1

This can help:
DECLARE #products TABLE
(
id INT ,
price MONEY ,
volume INT
)
DECLARE #discounts TABLE
(
id INT ,
Level INT ,
Factor MONEY
)
INSERT INTO #products
VALUES ( 1, 10, 35 ),
( 2, 15, 200 )
INSERT INTO #discounts
VALUES ( 1, 0, 1 ),
( 2, 25, 0.95 ),
( 3, 50, 0.90 ),
( 4, 100, 0.75 )
SELECT p.id, p.price * SUM(ca.m)
FROM #products p
CROSS APPLY ( SELECT * ,
Factor * ( -Level + LEAD(Level) OVER ( PARTITION BY p.id ORDER BY Level, d ) ) AS m
FROM ( SELECT 1 AS d ,
Level ,
Factor
FROM #discounts
WHERE Level < p.volume
UNION ALL
SELECT 2 AS d ,
p.volume ,
0
) t
) ca
GROUP BY p.id, p.price
Without grouping it returns:
id price volume d Level Factor m
1 10.00 35 1 0 1.00 25.00
1 10.00 35 1 25 0.95 9.50
1 10.00 35 2 35 0.00 NULL
2 15.00 200 1 0 1.00 25.00
2 15.00 200 1 25 0.95 23.75
2 15.00 200 1 50 0.90 45.00
2 15.00 200 1 100 0.75 75.00
2 15.00 200 2 200 0.00 NULL
Then just group by product and sum of m results in:
id Total
1 345.00
2 2531.25

Answer 2

For a given Volume and Price you can get the discount based on interval using LEAD which is available in SQL Server 2012+ onwards.
Sample Data
DECLARE #PriceTable TABLE(Volume INT,Price DECIMAL(9,2) )
DECLARE #Discount TABLE(StartLevel int, DiscountFactor DECIMAL(9,2))
INSERT INTO #PriceTable
VALUES(75, 20.5),
(150, 20),
(250, 20.5),
(0, 15);
INSERT INTO #Discount
VALUES(0, 1),
(25, 0.95),
(50, 0.90),
(100, 0.75);
Query
SELECT Volume,Price,FinalPrice
FROM #PriceTable P
CROSS APPLY(
SELECT SUM(CASE WHEN (MaxLevel >=StartLevel) THEN (MaxLevel-StartLevel) ELSE 0 END *DiscountFactor)*P.Price as FinalPrice
FROM
(
SELECT CASE WHEN LEAD(StartLevel)OVER(ORDER BY StartLevel) < P.Volume THEN LEAD(StartLevel)OVER(ORDER BY StartLevel) ELSE P.Volume END MaxLevel,StartLevel, DiscountFactor
FROM #Discount
) IQ
)T
Output
Volume Price FinalPrice
75 20.50 1460.6250
150 20.00 2625.0000
250 20.50 4228.1250
0 15.00 0.0000