You data model makes reporting more difficult if you want to list each transaction. If you only want one entry for each project with a total project cost and total project income, that would be fairly easy to do with what you have.
How much detail do you want in your "overview"?
I have for every project about 25 items for the costs and about 15 items for income. Every item has a categorie, the note and the sum. In the overview it´s similar. For example I want to see, that for advertising we plan in 6 projects the sum around 1.500€, so the total sum is 9.000€. This sum I want to compared with the planed budget, which is may be 8.000€. Thats all!
But you see, not only once the total for all costs.
Hear a picture from a project calculation.
Do want to list the project followed by one row for each category with a subtotal for each category?
Or do you want just one total for each category across many projects?
I would rethink your diagram, instead of having Kosten_Ein und Kosten_aus, just have Kosten.
Inside the Kosten table you can have a discriminatory field Ein / Aus.
In a report you can sort by this field, so you will have let's say alle Kosten_Ein first, then all the Aus.
You can have fitered portals by this field which will separe the 2 Kosten categories.
For a total sum you can have Real_Cost = Kost * If (Ein; 1; -1)
Thanks, I have to think again - it sound logic.
The report you describe is often called a "cross tab" report. If you search this site, you can find several different ways to produce such a report.