Field with TOTAL from Related Table (SUM, COUNT, AVERAGE, etc)

It would be nice if we could create fields that would have totals from related tables.

The totals could be SUM, COUNT, AVERAGE (or some other thing I did not think about)
And you could select which column of the related table you would use and a column of that table to apply a filter.

So let’s say I have a table of clients.
Each client has several orders. Each order several items.

So I have this order with 6 items (it’s a related table, right?), total of U$1000
When seeing the table of all orders, it would be important to see the number of items (count) of each order as well as the total of each order.

Then when seeing the clients table, it would be important to have a column showing the number of orders of that client, the total value of orders of that client, and the average of orders.