What are the total scores of body builder?
select total from body_builder
How about the average value of those scores?
select avg(total) from body_builder
What are the average scores of those results where the people weight is larger than 100.0?
select avg(t1.total) from body_builder as t1 join people as t2 on t1.people_id = t2.people_id where t2.weight > 100.0
What about the people height larger than 200?
select avg(t1.total) from body_builder as t1 join people as t2 on t1.people_id = t2.people_id where t2.height > 200