Hello Christophe,
at least there are two options:
1) Create a scripted calculation view in which you can use your "complex" join condition via SQL.
2) Create a graphical calculation view which
- joins both tables via the Area
- creates a calculated column on the joined values which calculates if the Invoice Date lies between Valid From/Valid To from the DISC_RATE table -> the result can be e.g. 0 = false, 1 = true
- filters on the calculated column to just return the joined values with the matching date (e.g. calculated column = 1).
Best Regards,
Florian