Which pictures are taken by a lens of brand 'Sigma'?
select * from photos as t2 join camera_lens as t3 on t2.camera_lens_id = t3.id where t3.brand = 'Sigma'
Show the name and prominence of the mountains whose picture is not taken by a lens of brand 'Sigma'.
select name, prominence from mountain except select t1.name, t1.prominence from mountain as t1 join photos as t2 on t1.id = t2.mountain_id join camera_lens as t3 on t2.camera_lens_id = t3.id where t3.brand = 'Sigma'