summaries of portal field

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This problem involves three databases.

One is a purchase order data base. The second is a line item database for the purchase orders. The third is a project database.

The way it is set up now, the first database (purchase orders) has repeating fields for the items purchased. But what I want to do is create another database, the second one only dealing with the line items and is related by the purchase order number. Now because one order can have items pertaining to multiple projects, I want to make a portal of these line items in the second database in both the purchase order database and the project data base so each item from any purchase order can be broken up and displayed by either project for the project database, or by purchase order number in that database.

The problem is I don't know how to obtain totals from a field from multiple records in a portal, ie extended price. Thank you for any help.