Counting

You are interested in figuring out where the customers of eSymphony are ordering from. Specifically, you want to know from how many different U.S.A. orders are coming in.

To calculate this you will need to include a filter to your data before applying the correct group function. Let's start building this query!

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Count the number of (non-missing) billing cities in the Invoice table.
Filter on invoices where the billing country is the USA.