CREATE TABLE student (id CLOB, name CLOB, dept_name CLOB, tot_cred NUMBER)
CREATE TABLE department (dept_name CLOB, building CLOB, budget NUMBER)
CREATE TABLE instructor (id CLOB, name CLOB, dept_name CLOB, salary NUMBER)
CREATE TABLE advisor (s_id CLOB, i_id CLOB)
CREATE TABLE takes (id CLOB, course_id CLOB, sec_id CLOB, semester CLOB, year NUMBER, grade CLOB)
CREATE TABLE section (course_id CLOB, sec_id CLOB, semester CLOB, year NUMBER, building CLOB, room_number CLOB, time_slot_id CLOB)
CREATE TABLE course (course_id CLOB, title CLOB, dept_name CLOB, credits NUMBER)
CREATE TABLE time_slot (time_slot_id CLOB, day CLOB, start_hr NUMBER, start_min NUMBER, end_hr NUMBER, end_min NUMBER)
CREATE TABLE classroom (building CLOB, room_number CLOB, capacity NUMBER)
CREATE TABLE prereq (course_id CLOB, prereq_id CLOB)
CREATE TABLE teaches (id CLOB, course_id CLOB, sec_id CLOB, semester CLOB, year NUMBER)

Find the id of instructors who didn't teach any courses?
SELECT id FROM instructor EXCEPT SELECT id FROM teaches
SELECT "id" FROM "instructor" EXCEPT SELECT "id" FROM "teaches"