Using Queries to Calculate Values
One popular use for database tables is to maintain sales records with fields for the order identifier, the product ordered, and the product’s price. What you can’t do in a table is perform a calculation—the fields are just designed to hold data. In a query, however, you can find totals, averages, or even the minimum or maximum value in the records found by your query.
To select fields from other tables for use in a calculation, double-click the Tables icon in the left pane of the Expression Builder, double-click the table with the target field, and then double-click the field name in the center pane.