CREATE TABLE aircraft (aid NUMBER, name CLOB, distance NUMBER)
CREATE TABLE employee (eid NUMBER, name CLOB, salary NUMBER)
CREATE TABLE flight (flno NUMBER, origin CLOB, destination CLOB, distance NUMBER, departure_date TIME, arrival_date TIME, price NUMBER, aid NUMBER)
CREATE TABLE certificate (eid NUMBER, aid NUMBER)

What are the ids of all employees that don't have certificates?
SELECT eid FROM employee EXCEPT SELECT eid FROM certificate
SELECT "eid" FROM "employee" EXCEPT SELECT "eid" FROM "certificate"