You need to add a new relationship to what you already have.
Open Manage | Database | Relationships.
Click Purchases to select it, then click the button with two green plus signs to make a new table occurrence of purchases.
Link this new table occurrence to customers like this:
Customers::CustomerID x Purchases 2::CustomerID
To use x instead of =, double click the line linking these two Table Occurrences and use the drop down to change the = to x. This operator matches all records in Customers to all records in Purchases 2.
Now you can add the summary field to your customers layout by selecting it from Purchases 2 or you can define a calculation field in customers as Sum ( Purchases 2::Amount )
If Table Occurrence is a new term, you might want to read this thread to learn more:
Thank you for this precise and easy to follow answer. There was one slight glitch that I worked through, but I am not sure it is the proper way to do it. I forgot to mention that I am not using a single field (like an ID#) as the match but am using two fields to make the match: Each table has the fields, Firstname and Lastname and this is how I formed the relationship in the original Table Occurances connecting both fields with an "=".
I did everything you said (but using both fields for the match between CUSTOMERS and the new PURCHASES 2) but I could not get the = in the connecting line to turn to an x. It made an x with the sides closed like an infinity sign. With this configuration, a summary or calculation field using PURCHASERS 2 still only showed each customer's individual total. In the "Edit relationship window it showed:
Customers Purchases 2
Firstname x Firstname
Lastname = Lastname
I could not find a way to connect the Lastnames also with a "x" so I decided to try deleting the link between the Lastnames (leaving only the link between the Firstnames) and this worked as you said it would.
I guess my question is this: Is this the proper way to acheive my goal and is the integrety of the database ok? Or should I be using both the Firstname and Lastname fields as relationship matches and find a way to make both fields display an "x."
Thank you again for your help in this. Milo
Keep in mind that this is a separate relationship to be added to what you already have. You shouldn't modify any that you've already created as that could affect the way parts of the system that you've already defined work. That said, there's no need for more than one pair of fields when you use the x operator as this operator matches all records from purchases with all records from customers. Any additional field pairs wouldn't make sense here.
Lastly, I strongly recommend that you dispense with using name fields to link customers to purchases. Names are not unqiue, they get changed from time to time and sometimes you enter a name incorrectly and don't discover the error right away. In all of those cases, changing the name fields in customers will disconnect your purchases records from that record. Define a serial number field in Customers and link it to a number field in Purchases to avoid all of those issues.