For each payment method, count the number of customers who use it.
select count(*) from customers group by payment_method
What is the most frequently used payment method?
select payment_method from customers group by payment_method order by count(*) desc limit 1
Return the name of the customers who use this payment method.
select customer_name from customers where payment_method = (select payment_method from customers group by payment_method order by count(*) desc limit 1)