Calculating de-cumulatived values in TSQL? - sql

Question

Given a table of cars and their odometer reading at various dates (first of each month), how can I write TSQL (ideally, for use as a SQL Server view) to return the "incremental" values?
In other words, I want the reverse operation from Calculate a Running Total in SQL Server.
Example:
On this table:
CarId | Date | Mileage
---------------------------
1 1/1/2000 10000
1 2/1/2000 11000
1 3/1/2000 12000
2 1/1/2000 10000
2 2/1/2000 11001
2 3/1/2000 12001
3 1/1/2000 10000
(missing datapoint for (3, 2/1/2000))
3 3/1/2000 12000
We'd return something like (the details/edge cases are flexible):
CarId | Date | Delta
---------------------------
1 1/1/2000 10000
1 2/1/2000 1000
1 3/1/2000 1000
2 1/1/2000 10000
2 2/1/2000 1001
2 3/1/2000 1000
3 1/1/2000 10000
3 3/1/2000 2000

Answer 1

This should work for SQL 2005 or higher:
WITH cteData As
(
SELECT
CarId,
Date,
Mileage,
ROW_NUMBER() OVER (PARTITION BY CarId ORDER BY Date) As RowNumber
FROM
dbo.Cars
)
SELECT
C.CarId,
C.Date,
CASE
WHEN P.CarId Is Null THEN C.Mileage
ELSE C.Mileage - P.Mileage
END As Delta
FROM
cteData As C
LEFT JOIN cteData As P
ON P.CarId = C.CarId
And P.RowNumber = C.RowNumber - 1
ORDER BY
C.CarId,
C.Date
;
SQL Fiddle
NB: This assumes that "missing datapoint for (3, 2/1/2000)" means that there is no row in the table for car 3, February 2000.

Answer 2

Same approach as the one from #Richard Deeming, but this one regards possible null values as included in original question.
;with cte ( rn, id, date, mileage )
as
(
select
row_number() over ( partition by id order by id, date )
, id
, date
, mileage
from
cars
where
mileage is not null
)
select
"current".id
, "current".date
, delta = isnull( "current".mileage - predecessor.mileage, "current".mileage )
from
cte as "current"
left join cte as predecessor
on "current".id = predecessor.id
and "current".rn - 1 = predecessor.rn
See SQL-Fiddle.

Answer 3

Trying to do this without dependence on any 2012 functions, cursor, while loop, etc.
This works within some limitation -- namely, the null-entry for car#3's entry is a problem for it:
DECLARE #cars table ([id] int, [date] smalldatetime, [mileage] int)
INSERT INTO #cars ([id], [date], [mileage])
SELECT 1, '1/1/2000', 10000 UNION ALL
SELECT 1, '2/1/2000', 11000 UNION ALL
SELECT 1, '3/1/2000', 12000 UNION ALL
SELECT 2, '1/1/2000', 10000 UNION ALL
SELECT 2, '2/1/2000', 11000 UNION ALL
SELECT 2, '3/1/2000', 12000 UNION ALL
SELECT 3, '1/1/2000', 10000 UNION ALL
SELECT 3, '2/1/2000', NULL UNION ALL
SELECT 3, '3/1/2000', 12000
SELECT t1.id, t1.date, t1.mileage, t2.id, t2.date, t2.mileage, t1.mileage - t2.mileage as miles FROM #cars t1
LEFT JOIN #cars t2
ON t1.id = t2.id
AND t1.date = DATEADD(MONTH,1, t2.date)

Answer 4

Window functions are great. But SQL Server does not have the one you need until SQL Server 2012. There, you have the lag function:
select t.*,
(Milage - lag(Milage) over (partition by carId order by date)) as Delta
from t
For earlier versions, you can use a correlated subquery:
[trouble uploading query], alas.
select t.*, (Mileage - prevMileage) as Delta
from (select t.*,
(select top 1 Mileage from t t2
 where t2.carId = t.carId and t2.date < t.date order by desc
 ) as prevDelta
from t
) t