T-SQL Max Weight Query - sql-server

Question

I have a table with these columns:
BatchNumber, BagNumber, BagWeight, CumulativeWeight
Each batch can have up to 30 bags and the other columns are self-explanatory.
What I need is a query which finds the maximum cumulative weight for each batch, here is what I have so far.
DECLARE #HighestBagNumber INT;
DECLARE #BatchNumber CHAR(8);
SET #BatchNumber = 37708;
SELECT #HighestBagNumber = MAX(BagNumber)
FROM FSD3BagLog
WHERE BatchNumber = #BatchNumber
SELECT BatchNumber, BagNumber, CumulativeWeight
FROM FSD3BagLog
WHERE BagNumber = #HighestBagNumber
AND BatchNumber = #BatchNumber
This works for one batch at a time but I need it to look at all batches in the table. As you might be able to tell, I am a total beginner so please be as critical as you want, its all good.

Answer 1

Yes GROUP BY seems right, with the proper storage model it should be:
SELECT BatchNumber, COUNT(BagNumber), SUM(BagWeight)
FROM FSD3BagLog
GROUP BY BatchNumber
Result: 100, 30, 600
(Where batch number = 100, there are 30 per batch, and weight of each bag = 20)
But based on you current working query it looks like you are storing denomalizing the data and storing cumulative weight as you go, probably using triggers or some other code that fires when the table is updated.
So if cumulative weight represents the total weight of a given batch, you can get rid of it and use the query above.
If cumulative weight is something else, such as total of all bags up to a certain point in time, you can still get rid of it. In that case you would simple do something like:
SELECT BatchNumber, SUM(BagWeight) AS CumulativeWeight
FROM FSD3BagLog
WHERE ModifiedDate <= '2018-08-11 06:18:00'
Given you are storing ModifiedDate as a column on your table, this will give you the cumulative weight of all bags up to today at 6:18 AM.

Answer 2

Simple GROUP BY should do the job:
SELECT BatchNumber, MAX(CumulativeWeight)
FROM my_table
GROUP BY BatchNumber

Answer 3

with batches_ranked as
(
select BatchNumber, BagNumber,
CumulativeWeight = sum(Weight) over (partition by BatchNumber order by BagNumber),
[Rank] = row_number() over (partition by BatchNumber order by BagNumber desc)
from FSD3BagLog
)
select * from batches_ranked where [Rank] = 1
sounds like you have CumulativeWeight stored in the table, if that is always increasing with BagNumber then you can simplify the query to just:
select BatchNumber, max(BagNumber), max(CumulativeWeight)
from FSD3BagLog group by BatchNumber