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

How many different users wrote some reviews?
SELECT COUNT(DISTINCT u_id) FROM review
SELECT COUNT(DISTINCT "u_id") FROM "review"