Lennart Hagemann wrote:
...Now I would like to index those 3 orders by order date - the other 2 orders are set to NULL. This definition should work for every order/customer and is saved permanently inside the datasource/can be used in the whole workbook. Is this scenario possible with tableau?
This is the crux of your problem.
You can do that indexing. But that takes a table calc to do it.
And a table calc is only evaluated in the context of the underlying table of the sheet on which it is being used.
If you want those numbers used in other sheets, you'll have to re-evaluate them there. (It means using the same table calc, but requires having the same data in the underlying table for that sheet.)
Tableau doesn't write to the data source. Data flow is only one direction -- into tableau.