How many customers are there?
select count ( * ) from customer
How many are there that has made at least one payment?
select count ( * ) from ( select * from customer as t1 join payment as t2 on t1.customer_id = t2.customer_id group by t1.customer_id )
Yes please
select t1.first_name, t1.last_name, t1.customer_id from customer as t1 join payment as t2 on t1.customer_id = t2.customer_id group by t1.customer_id order by sum ( amount ) asc limit 1
How much money did that customer spend?
select sum ( amount ) from customer as t1 join payment as t2 on t1.customer_id = t2.customer_id group by t1.customer_id order by sum ( amount ) asc limit 1