Calculation with a selected SQL Database

[Dima1989]

Hello community,

I'm at the very beginning working with qlik and also it's the first time I'm using this forum hoping its the right place for my problem.

I have the following chart and would now like to add a third line. This line should be calculated from the two existing data. 

The formula is like:

=WENN(L80-(K80-K81)>0;L80-(K80-K81);0)

So, I have two data streams (one from an sql "abarbeitung" and the other from an excel "Füllstände") matching them with the Date. I need to connect the streams with an formula which should do the operation with data of two different dates. I have absolutely no idea how get get the solution for that requirement.

If you need more input from me tell me what you need.

please help me. thx.