Here are a couple interesting things you can do. Let's say you've got a bunch of customers with a few invoices each, and you just want to know how much each of them has spent. You take your one customer record and you add many invoice records together to get the total they spent on all invoices.
You can also multiply items: let's say your invoice table has the item price, and number of items purchased on the invoice, but not the total per invoice. You can multiply item price by number of items to get the total.
You'll get a window where you can choose which table to use. In this example I'll use the invoices table. Click Add to add the table.
Then double-click a few fields to add them. I'm going to add a few here, then delete some later.
Now I've got the fields added.
If I click Run Query then I just get those fields.
Clicking Run Query gives the results.
Which look like this--you get the Alias you typed, not the multiplication. Note however that it is not formatted as currency in this example. If you want to change it you can right-click on the column heading, choose Column Format, and select a currency or number format.
And here's what that looks like.
Here's a note. You don't have to have the fields you're multiplying in the query. You can unmark the checkmarks as shown and they won't show up. Or you don't even have to have them at all; you can right-click on any column head and choose Delete to remove it from the query. (Do this in the bottom area of the query where you do the defining, not the top where you see the results.)
Now you're thinking about doing that sum thing, adding up all the totals for all the multiple invoice for each single customer. For this, you need to have just the customer ID (one) and the calculated field (many for each one customer). So I deleted everything but those two columns.
Then for the single field I selected Group in the function list, and I select Sum for the many field, as shown.
Run query, and here are the results. Note that now each customer is only listed once, and the total for their invoices is calculated.