For all the invoices, show the invoice ids and customer ids!
select id, customer_id from invoices;
How many have each customer ordered!
select t1.id, count(*) from customers as t1 join invoices as t2 on t1.id = t2.customer_id group by t1.id;
Show their names for each!
select t1.first_name, t1.last_name, t1.id, count(*) from customers as t1 join invoices as t2 on t1.id = t2.customer_id group by t1.id;
Show the number for customer named Lucas Mancini!
select count(*) from customers as t1 join invoices as t2 on t1.id = t2.customer_id where t1.first_name = "Lucas" and t1.last_name = "Mancini";