Which mountains are in the country 'Ethiopia'?
select * from mountain where country = 'Ethiopia'
Which camera lenses are used to take photos of these mountains? Return the camera lens ids.
select t2.camera_lens_id from mountain as t1 join photos as t2 on t1.id = t2.mountain_id where t1.country = 'Ethiopia'
How many distinct kinds of camera lenses are there, among these?
select count(distinct t2.camera_lens_id) from mountain as t1 join photos as t2 on t1.id = t2.mountain_id where t1.country = 'Ethiopia'