Subtotaling Multiple Records from a different DB

Subtotaling Multiple Records from a different DB

In Zoho Creator, I have a Project DB with each project having a unique ID. I have another DB for Price Estimates of various elements of a given project-- each estimate is entered separately as a record in the Price Estimate DB with a Project ID pulled from the Project DB.

So, in the Estimate DB I have several records with the same Project ID. How can I have the SUBTOTAL of all the Estimate records with the same Project ID appear in a field in the Project DB?

Thank you!