What are the different states in the US that have invoices?
select distinct billing_state from invoices where billing_country = "USA";
Which one has the least number of invoices?
select billing_state, count(*) from invoices where billing_country = "USA" group by billing_state order by count(*) asc limit 1;
How about the most?
select billing_state, count(*) from invoices where billing_country = "USA" group by billing_state order by count(*) desc limit 1;