Which customer ids took lessons?
select distinct customer_id from lessons
Sum over the lesson time for those groups. Also count the number of lessons take.
select customer_id, sum(lesson_time), count(*) from lessons group by customer_id
What is the customer id of Rylan Goodwin?
select customer_id from customers where first_name = "Rylan" and last_name = "Goodwin";
How long is the total lesson time took by customer with first name as Rylan and last name as Goodwin?
select sum(t1.lesson_time) from lessons as t1 join customers as t2 on t1.customer_id = t2.customer_id where t2.first_name = "Rylan" and t2.last_name = "Goodwin";