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

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