Incrementing a column for each rows from two queries - sql-server-2014

Question

I have two queries that give respectively the number of working unit bought, and the number of working unit consumed by a client.
I am working on a SQL Server 2014
The WUBought query returns something like this example :
Customer Year Month UnitBought
Cust1 2015 6 50
Cust2 2014 7 100
Cust1 2013 10 30
Cust3 2015 2 40
The other query returns the number that were consumed by a client :
Customer Year Month UnitConsumed
Cust1 2015 2 6
Cust1 2015 5 20
Cust2 2015 3 8
Cust1 2015 4 3
Cust3 2015 2 10
What I am basically trying to do, is a sum of what has been bought for every month, minus what has been consumed. Here is an example of what I want as a result for the first six months for Cust1 :
Customer Year Month Remaining
Cust1 2015 1 30
Cust1 2015 2 24
Cust2 2015 3 24
Cust1 2015 4 21
Cust3 2015 5 1
Cust3 2015 6 51
The query that returns the WU bought with a UNION ALL from a table that lists every month, to get each month even if there is no value :
SELECT Customer, [Year], [Month], SUM(UOBought) AS UORest
FROM WU_Bought
GROUP BY [Customer], [PurchaseDate]
UNION ALL
SELECT '' AS Customer, [Year], [Month], '' AS UORest
FROM Months
GROUP BY [Year], [Month]
Here is the query that sums every bought unit every month, with the same union statement :
SELECT Customer, [Year], [Month], SUM(TotalConsumed) * -1 AS UORest
FROM WUConsumed
GROUP BY Customer, Year, Month
UNION ALL
SELECT '' AS Customer, [Year], [Month], '' AS UORest
FROM EveryMonths
GROUP BY Year, Month
Right now I think I must adjust the first one, forcing it to keep the previous sum, but I am not sure how I can do that.

Answer 1

Does this work for you?
SELECT b.customer_id, b.year, b.month, SUM(b.units_bought) AS units_bought, ISNULL(c.units_consumed,0) AS units_consumed, SUM(b.units_bought) - ISNULL(c.units_consumed,0) AS units_remaining
FROM Bought b
LEFT JOIN Consumed c
ON b.customer_id = c.customer_id AND b.year = c.year AND b.month = c.month
GROUP BY b.customer_id, b.year, b.month

Answer 2

Ok, I got it working.
What I did was really "simple", using a SQL Server feature, available since 2012 :
ROWS UNBOUNDED PRECEDING
Here is a pretty clear article about this feature.
I created an other view grouping the results from the queries about consumed and bought units with a UNION ALL clause, called "WU_Closing_View", then used the ROWS UNBOUNDED PRECEDING within it :
SELECT Customer, Year, Month, SUM(Closing) OVER(PARTITION BY Customer ORDER BY Year, Month ROWS UNBOUNDED PRECEDING) AS Closing
FROM WU_Closing_View
GROUP BY Customer, Year, Month, Closing
UNION ALL
SELECT '' AS Customer, Year, Month, '' AS Sum_bought
FROM Months
GROUP BY Year, Month
ORDER BY Customer, Year, Month
Note that I used PARTITION BY, in order to sum by client. Because I wanted to show every month in a SSRS matrix, I added a "UNION ALL" pointing to a table that has every year and month on it for an empty client, from 2010 to 2017. But it is optional if you don't need the evolution for every month.
There may be an easier way, but that's the one I found so far.