CREATE TABLE review (a_id DECIMAL, u_id DECIMAL, i_id DECIMAL, rating DECIMAL, rank DECIMAL)
CREATE TABLE item (i_id DECIMAL, title TEXT)
CREATE TABLE useracct (u_id DECIMAL, name TEXT)
CREATE TABLE trust (source_u_id DECIMAL, target_u_id DECIMAL, trust DECIMAL)

How many different items were reviewed by some users?
SELECT COUNT(DISTINCT i_id) FROM review
SELECT COUNT(DISTINCT "i_id") FROM "review"