Sum of field in join table depending on both parent tables
I have the common case of two tables called Invoices and Products.
Invoices and Products have a many-to-many relation. The are connected via a Lines table. Lines have foreign keys from both Products och Invoices.
Lines has a field called LineTotalPrice (which is the sum of the Line fields LineQuantity and LinePrice)
Products has a field called "Tax rate"
Now I want to add a calculated field in Invoice that sums up LineTotalPrice for all Lines that matching a certain Invoice AND matching Products having a certain tax rate (say 25%).
How to do this? I guess I have to add a constant "25" somewhere and at least one more table ocurrance of Lines or Products but I can't get it right... Can someone help me?