Calculating a difference for an item, between most recent dates - sql

Question

I'm looking to calculate the difference between weights of my loaded trucks everyday. Basically, I want to know the incremental weight amounts carried each day. In theory, the trucks will be running on a daily basis, but I'm using a simplified data set on my mock database.
This is the code I've come up with:
--create table dbo.truck
--(TruckID int, Weight float, datetrunc date, weightdiff float)
declare #dt1 datetime, #dt2 datetime
select #dt2 = max(datetrunc) from truck
select #dt1 = max(datetrunc) from truck where datetrunc < #dt2
select #dt1 [dt1], #dt2 [dt2]
SELECT t1.truckid, t2.weight - t1.weight [WeightDiff]
FROM truck t1
inner join truck t2 ON t1.truckid = t2.truckid
WHERE t1.datetrunc = #dt1
AND t2.datetrunc = #dt2
UPDATE truck SET WeightDiff = x.WeightDiff
FROM (
SELECT t1.truckid, t2.weight - t1.weight [WeightDiff]
FROM truck t1
inner join truck t2 ON t1.truckid = t2.truckid
WHERE t1.datetrunc = #dt1
AND t2.datetrunc = #dt2
) AS X
WHERE truck.datetrunc = #dt2
AND x.truckid = truck.truckid
SELECT t1.truckid, #dt2, t2.weight - t1.weight
FROM truck t1
inner join truck t2 ON t1.truckid = t2.truckid
WHERE t1.datetrunc = #dt1
AND t2.datetrunc = #dt2
I'm hoping for a difference between each date to show. However, it is only dsiplaying the difference between the latest dates, while deleting&null the other rows:
TruckID Weight datetrunc weightdiff
1 1000 2019-03-01 NULL
2 1111 2019-03-01 NULL
3 1222 2019-03-01 NULL
1 1050 2019-03-15 NULL
2 1700 2019-03-15 NULL
3 1400 2019-03-15 NULL
1 1125 2019-03-31 75
2 1725 2019-03-31 25
3 1600 2019-03-31 200
I want:
TruckID Weight datetrunc weightdiff
1 1000 2019-03-01 NULL
2 1111 2019-03-01 NULL
3 1222 2019-03-01 NULL
4 1400 2019-03-01 NULL
1 1050 2019-03-15 50
2 1700 2019-03-15 589
3 1400 2019-03-15 178
4 1490 2019-03-15 90
1 1125 2019-03-31 75
2 1725 2019-03-31 25
3 1600 2019-03-31 200
4 1900 2019-03-31 510
Notice how some TruckID 4 were completed deleted from my dataset. Also, how it replaces the weight difference.
I can't seem to figure how to make my data display properly. Any suggestions help, thanks!

Answer 1

For SQL Server 2008
select a.truckid, a.weight, a.datetrunc, (a.weight - c.weight) weightdiff
from truck a
outer apply (select top 1 weight from truck b where b.datetrunc<a.datetrunc and b.truckid=a.truckid order by b.datetrunc desc) c
Newer SQL Server
select truckid, weight, datetrunc, weight - lag(weight) over (partition by truckid order by datetrunc) weightdiff
from truck