I need to sum the mileage ran each month per vehicle - sql-server-2008-r2

Question

I have a series of vehicles that are fueled every day, entering the total fuel and the odometer reading. I need to sum the mileage ran each month per vehicle.
My table is fuel, vehicle is bus, date is service_date, odometer reading is mileage, fuel entered is quantity.
So I would take the mileage from the first of every month and subtract that from the first of the previous month per vehicle, in this case 3237. To make matters worse, if the vehicle was not fueled on the first, I would want the mileage from the last day it was fueled on the previous month.
service_date bus mileage quantity
7/1/2018 202 149654 34
7/3/2018 202 150256 40.5
7/4/2018 202 150562 42
7/6/2018 202 150853 41
7/7/2018 202 151191 37
7/8/2018 202 151323 23.6
7/15/2018 202 151502 39
7/13/2018 202 151806 45
8/1/2018 202 152891 37 3237

Answer 1

Not sure I understand all of your specific requirements, but here is how I would write a query to report on mileage for a fleet of buses.
First I would get the ending mileage for each month using GROUP BY and MAX like this:
SELECT bus_id
, MAX(bus_mileage) AS 'ending_mileage'
, DATEPART(YEAR, service_date) AS 'year'
, DATEPART(MONTH, service_date) AS 'month'
, MAX(service_date) AS 'last_service_date'
FROM #mileage
GROUP BY bus_id
, DATEPART(YEAR, service_date)
, DATEPART(MONTH, service_date)
Then I would use that as a subquery to pull the previous month's ending mileage. This would give you the starting mileage for the month.
SELECT bus_id
, ending_mileage
, (
SELECT MAX(bus_mileage)
FROM #mileage m
WHERE DATEPART(year, m.service_date) = DATEPART(YEAR, DATEADD(MONTH, -1, bus_mileage.last_service_date))
AND DATEPART(MONTH, m.service_date) = DATEPART(MONTH, DATEADD(MONTH, -1, bus_mileage.last_service_date))
AND m.bus_id = bus_mileage.bus_id
) AS 'starting_mileage'
, mileage_year
, mileage_month
FROM (
SELECT bus_id
, MAX(bus_mileage) AS 'ending_mileage'
, DATEPART(YEAR, service_date) AS 'mileage_year'
, DATEPART(MONTH, service_date) AS 'mileage_month'
, MAX(service_date) AS 'last_service_date'
FROM #mileage
GROUP BY bus_id
, DATEPART(YEAR, service_date)
, DATEPART(MONTH, service_date)
) bus_mileage
Then I would wrap the whole thing in an outer query that subtracts the starting mileage from the ending mileage.
SELECT bus_id
, ending_mileage
, starting_mileage
, ending_mileage - starting_mileage AS 'mileage_for_the_month'
, mileage_year
, mileage_month
FROM (
SELECT bus_id
, ending_mileage
, (
SELECT MAX(bus_mileage)
FROM #mileage m
WHERE DATEPART(year, m.service_date) = DATEPART(YEAR, DATEADD(MONTH, -1, bus_mileage.last_service_date))
AND DATEPART(MONTH, m.service_date) = DATEPART(MONTH, DATEADD(MONTH, -1, bus_mileage.last_service_date))
AND m.bus_id = bus_mileage.bus_id
) AS 'starting_mileage'
, mileage_year
, mileage_month
FROM (
SELECT bus_id
, MAX(bus_mileage) AS 'ending_mileage'
, DATEPART(YEAR, service_date) AS 'mileage_year'
, DATEPART(MONTH, service_date) AS 'mileage_month'
, MAX(service_date) AS 'last_service_date'
FROM #mileage
GROUP BY bus_id
, DATEPART(YEAR, service_date)
, DATEPART(MONTH, service_date)
) bus_mileage
) monthly_mileage
Here is the whole thing with demo data so you can see how it all works together.
DECLARE #mileage TABLE(
service_date DATE NOT NULL
, bus_id INT NOT NULL
, bus_mileage INT NOT NULL
)
INSERT INTO #mileage (service_date, bus_id, bus_mileage)
VALUES ('7/1/2018', 202, 149654)
, ('7/15/2018', 202, 151502)
, ('8/1/2018', 202, 152891)
, ('8/15/2018', 202, 153502)
, ('9/3/2018', 202, 154891)
, ('10/15/2018', 202, 155502)
, ('11/3/2018', 202, 157891)
, ('7/5/2018', 302, 155502)
, ('8/3/2018', 302, 157691)
SELECT bus_id
, ending_mileage
, starting_mileage
, ending_mileage - starting_mileage AS 'mileage_for_the_month'
, mileage_year
, mileage_month
FROM (
SELECT bus_id
, ending_mileage
, (
SELECT MAX(bus_mileage)
FROM #mileage m
WHERE DATEPART(year, m.service_date) = DATEPART(YEAR, DATEADD(MONTH, -1, bus_mileage.last_service_date))
AND DATEPART(MONTH, m.service_date) = DATEPART(MONTH, DATEADD(MONTH, -1, bus_mileage.last_service_date))
AND m.bus_id = bus_mileage.bus_id
) AS 'starting_mileage'
, mileage_year
, mileage_month
FROM (
SELECT bus_id
, MAX(bus_mileage) AS 'ending_mileage'
, DATEPART(YEAR, service_date) AS 'mileage_year'
, DATEPART(MONTH, service_date) AS 'mileage_month'
, MAX(service_date) AS 'last_service_date'
FROM #mileage
GROUP BY bus_id
, DATEPART(YEAR, service_date)
, DATEPART(MONTH, service_date)
) bus_mileage
) monthly_mileage
Not sure if this meets your requirements exactly, but this is how I would report on mileage based on the information you've given me.