Tell me how many payments are processed by MasterCard?
select count(*) from payments where payment_method_code = "MasterCard"
How about that of Discover Card?
select count(*) from payments where payment_method_code = "Discover Card"
With which kind of payment method were the least number of payments processed?
select payment_method_code from payments group by payment_method_code order by count(*) asc limit 1