CREATE TABLE photos (id VARCHAR2, camera_lens_id VARCHAR2)
CREATE TABLE camera_lens (id VARCHAR2, camera_lens_id VARCHAR2)

How many camera lenses are not used in taking any photos?
SELECT COUNT(*) FROM camera_lens WHERE NOT id IN (SELECT camera_lens_id FROM photos)
SELECT COUNT(*) FROM "camera_lens" WHERE NOT "id" IN (SELECT "camera_lens_id" FROM "photos")