I have an application(C#) that manages a stock level for every storage unit.
But now i want a complete overview of the stock level of every product from every stock unit together.
For example i have
where ProductID = '1'
If you want to combine, you want an aggregate function,
sum in your case:
select sum(StockLevel) from Supply where ProductID = '1'
and get the apples being summed up:
75 which is
50 + 25