CREATE TABLE reservations (code NUMBER, room CLOB, checkin CLOB, checkout CLOB, rate NUMBER, lastname CLOB, firstname CLOB, adults NUMBER, kids NUMBER)
CREATE TABLE rooms (roomid CLOB, roomname CLOB, beds NUMBER, bedtype CLOB, maxoccupancy NUMBER, baseprice NUMBER, decor CLOB)

How many rooms have not had any reservation yet?
SELECT COUNT(*) FROM rooms WHERE NOT roomid IN (SELECT DISTINCT room FROM reservations)
SELECT COUNT(*) FROM "rooms" WHERE NOT "roomid" IN (SELECT DISTINCT "room" FROM "reservations")